Fix/bedrock cache point messages

[af001]

Description

This PR fixes the cachePoint formatting issue in BedrockModel that was causing ParamValidationError when using prompt caching with system prompts.

Problem: When using cachePoint in system prompts, Bedrock's API returned a validation error because the system content blocks were not being formatted correctly. Bedrock requires cachePoint to be a separate content block (tagged union), not merged with other fields.

Solution: Added _format_bedrock_system_blocks() method that formats system content blocks using the same logic as message content blocks, ensuring cachePoint blocks remain as separate content blocks.

Verified: Cache metrics now show cacheWriteInputTokens on first request and cacheReadInputTokens on subsequent requests, confirming prompt caching works correctly.

Note: Bedrock expects

Related Issues

Fixes #1219

Documentation PR

N/A - No documentation changes needed

Type of Change

Bug fix

Testing

How have you tested the change?

• All 92 bedrock unit tests pass (hatch test -- tests/strands/models/test_bedrock.py)

• All 1608 unit tests pass (hatch test)

• All 19 bedrock integration tests pass (hatch run test-integ -- tests_integ/models/test_model_bedrock.py)

• Added 5 new tests specifically for cachePoint formatting

• Verified cache metrics show actual cache hits with real Bedrock API calls

• I ran hatch run prepare

Checklist

• I have read the CONTRIBUTING document
• I have added any necessary tests that prove my fix is effective or my feature works
• I have updated the documentation accordingly
• I have added an appropriate example to the documentation to outline the feature, or no new docs are needed
• My changes generate no new warnings
• Any dependent changes have been merged and published

Output

================================================================================
CACHE PERFORMANCE SUMMARY
================================================================================

Request 1 (First - cache CREATED):
  - Cache Write Input Tokens: 1761 tokens  <- system prompt written to cache
  - Cache Read Input Tokens:  0 tokens
  - Input Tokens:             14 tokens  <- user query only

Request 2 (Second - cache HIT):
  - Cache Write Input Tokens: 0 tokens
  - Cache Read Input Tokens:  1761 tokens  <- reused from cache!
  - Input Tokens:             197 tokens  <- user query + conversation

Request 3 (Third - cache HIT):
  - Cache Write Input Tokens: 0 tokens
  - Cache Read Input Tokens:  1761 tokens  <- reused from cache!
  - Input Tokens:             442 tokens  <- user query + conversation

================================================================================
SUCCESS: Prompt caching is working!
   Total Cache Write: 1761 tokens (charged at write rate)
   Total Cache Read:  3522 tokens (charged at ~90% discount)
   Total Input:       653 tokens (charged at standard rate)

   Without caching, requests 2 & 3 would have cost 3522 more input tokens!
================================================================================

Notes

NOTE: Bedrock Prompt Caching Minimum Token Requirements (5 minute cache)
--------------------------------------------------------
| Model                  | Min Tokens per Cache Checkpoint |
|------------------------|--------------------------------|
| Claude Sonnet 4        | 1,024 tokens                   |
| Claude 3.7 Sonnet      | 1,024 tokens                   |
| Claude 3.5 Haiku       | 2,048 tokens                   |
| Claude Opus 4.5        | 4,096 tokens                   |
| Amazon Nova (all)      | 1,000 tokens                   |
--------------------------------------------------------
Reference: https://docs.aws.amazon.com/bedrock/latest/userguide/prompt-caching.html