fix: distinguish NXDOMAIN from network errors in RDNS lookup

[cnlangzi]

Summary by Sourcery

Bug Fixes:

• Treat NXDOMAIN (missing PTR record) during reverse DNS lookup as a hard verification failure instead of a retryable network error.

[sourcery-ai]

Reviewer's guide (collapsed on small PRs)

Reviewer's Guide

Handle reverse DNS lookup errors by treating NXDOMAIN (no PTR record) as a hard failure while keeping other network errors as retriable.

Sequence diagram for RDNS verification with NXDOMAIN handling

sequenceDiagram
  participant Bot
  participant Net as net

  Bot->>Net: LookupAddr(ipStr)
  Net-->>Bot: error
  alt error is net.DNSError with IsNotFound true
    Bot->>Bot: fail.Add(ipStr)
    Bot-->>Bot: return StatusFailed
  else other network error
    Bot-->>Bot: return StatusPending
  end

Loading

File-Level Changes

Change	Details	Files
Differentiate NXDOMAIN from other network errors in reverse DNS verification and update status/fail-cache behavior accordingly.	Inspect net.LookupAddr error for *net.DNSError and check IsNotFound to detect NXDOMAIN (no PTR record). On NXDOMAIN, mark the IP as failed via b.fail.Add(ipStr) and return StatusFailed to indicate a hard failure. For all other lookup errors (timeouts, transient network issues), continue returning StatusPending to allow retries.	bot.go

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[sourcery-ai]

Hey - I've left some high level feedback:

• Consider using errors.As instead of a direct type assertion to *net.DNSError so that wrapped DNS errors are still correctly classified as NXDOMAIN vs network issues.

Prompt for AI Agents

Please address the comments from this code review:

## Overall Comments
- Consider using `errors.As` instead of a direct type assertion to `*net.DNSError` so that wrapped DNS errors are still correctly classified as NXDOMAIN vs network issues.

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[codecov]

Codecov Report

❌ Patch coverage is 0% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 71.56%. Comparing base (60ce193) to head (c9cd856).
⚠️ Report is 4 commits behind head on main.

Files with missing lines	Patch %	Lines
bot.go	0.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #13      +/-   ##
==========================================
- Coverage   72.76%   71.56%   -1.21%     
==========================================
  Files          15       17       +2     
  Lines         661      735      +74     
==========================================
+ Hits          481      526      +45     
- Misses        136      155      +19     
- Partials       44       54      +10     

☔ View full report in Codecov by Sentry.
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[github-actions]

Benchmark Results

BenchmarkFindBotByUA_Hit_First             	  774824	      1714 ns/op	      21 B/op	       0 allocs/op
BenchmarkFindBotByUA_Hit_First-4           	 2457927	       694.6 ns/op	      17 B/op	       0 allocs/op
BenchmarkFindBotByUA_Hit_First-8           	 3359643	       612.4 ns/op	      15 B/op	       0 allocs/op
BenchmarkFindBotByUA_Hit_Middle            	 1427029	       877.4 ns/op	      13 B/op	       0 allocs/op
BenchmarkFindBotByUA_Hit_Middle-4          	 4100294	       499.8 ns/op	      13 B/op	       0 allocs/op
BenchmarkFindBotByUA_Hit_Middle-8          	 5023794	       333.2 ns/op	       8 B/op	       0 allocs/op
BenchmarkFindBotByUA_Hit_Last              	 1408330	      1207 ns/op	      19 B/op	       0 allocs/op
BenchmarkFindBotByUA_Hit_Last-4            	 2709805	       397.3 ns/op	      11 B/op	       0 allocs/op
BenchmarkFindBotByUA_Hit_Last-8            	--- FAIL: BenchmarkFindBotByUA_Hit_Last-8
BenchmarkFindBotByUA_Miss                  	  507398	      2635 ns/op	      95 B/op	       0 allocs/op
BenchmarkFindBotByUA_Miss-4                	 1363532	       858.5 ns/op	      21 B/op	       0 allocs/op
BenchmarkFindBotByUA_Miss-8                	 1342603	       901.3 ns/op	      25 B/op	       0 allocs/op
BenchmarkFindBotByUA_CaseSensitive         	 1000000	      1020 ns/op	      13 B/op	       0 allocs/op
BenchmarkFindBotByUA_CaseSensitive-4       	 3121101	       444.3 ns/op	      12 B/op	       0 allocs/op
BenchmarkFindBotByUA_CaseSensitive-8       	 4787684	       307.5 ns/op	       6 B/op	       0 allocs/op
BenchmarkValidate_KnownBot_IPHit           	 1000000	      1092 ns/op	      23 B/op	       0 allocs/op
BenchmarkValidate_KnownBot_IPHit-4         	 2200286	       560.4 ns/op	      14 B/op	       0 allocs/op
BenchmarkValidate_KnownBot_IPHit-8         	 3134862	       526.5 ns/op	      20 B/op	       0 allocs/op
BenchmarkValidate_Browser                  	--- FAIL: BenchmarkValidate_Browser
BenchmarkValidate_Browser-4                	  627133	      2057 ns/op	      77 B/op	       0 allocs/op
BenchmarkValidate_Browser-8                	  498900	      2026 ns/op	      79 B/op	       0 allocs/op
BenchmarkContainsWord                      	72368844	        18.29 ns/op	       0 B/op	       0 allocs/op
BenchmarkContainsWord-4                    	73669558	        16.87 ns/op	       0 B/op	       0 allocs/op
BenchmarkContainsWord-8                    	73214562	        16.20 ns/op	       0 B/op	       0 allocs/op
BenchmarkValidate_WithBotUA                	  952285	      1165 ns/op	       9 B/op	       0 allocs/op
BenchmarkValidate_WithBotUA-4              	 2747359	       440.8 ns/op	      10 B/op	       0 allocs/op
BenchmarkValidate_WithBotUA-8              	 2728572	       451.9 ns/op	       4 B/op	       0 allocs/op
BenchmarkValidate_WithBotUA_IPMismatch     	  773910	      1979 ns/op	      65 B/op	       0 allocs/op
BenchmarkValidate_WithBotUA_IPMismatch-4   	 2018013	       577.5 ns/op	      18 B/op	       0 allocs/op
BenchmarkValidate_WithBotUA_IPMismatch-8   	 2027438	       571.5 ns/op	      10 B/op	       0 allocs/op
BenchmarkValidate_BrowserUA                	  287529	      4971 ns/op	     200 B/op	       1 allocs/op
BenchmarkValidate_BrowserUA-4              	--- FAIL: BenchmarkValidate_BrowserUA-4
BenchmarkValidate_BrowserUA-8              	  854090	      1521 ns/op	      56 B/op	       0 allocs/op
BenchmarkValidate_UnknownBotUA             	 8143225	       186.2 ns/op	       7 B/op	       0 allocs/op
BenchmarkValidate_UnknownBotUA-4           	21948859	        60.53 ns/op	       3 B/op	       0 allocs/op
BenchmarkValidate_UnknownBotUA-8           	22563117	        56.80 ns/op	       1 B/op	       0 allocs/op
BenchmarkContainsIP                        	55834134	      1962 ns/op	       1 B/op	       0 allocs/op
BenchmarkContainsIP-4                      	94331160	        11.27 ns/op	       0 B/op	       0 allocs/op
BenchmarkContainsIP-8                      	100000000	        11.56 ns/op	       0 B/op	       0 allocs/op
BenchmarkFindBotByUA                       	  795450	      1943 ns/op	      73 B/op	       0 allocs/op
BenchmarkFindBotByUA-4                     	 2041138	       607.5 ns/op	      21 B/op	       0 allocs/op
BenchmarkFindBotByUA-8                     	 2034795	       578.4 ns/op	      17 B/op	       0 allocs/op
BenchmarkClassifyUA                        	 2052313	       550.0 ns/op	       4 B/op	       0 allocs/op
BenchmarkClassifyUA-4                      	SIGQUIT: quit