chore: Refactor CometScanRule to improve scan selection and fallback logic

[andygrove]

Which issue does this PR close?

N/A

Rationale for this change

Refactoring to simplify scan selection and fallback logic

What changes are included in this PR?

No functional changes. Just refactoring.

How are these changes tested?

Existing tests.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 69.04762% with 13 lines in your changes missing coverage. Please review.
✅ Project coverage is 59.54%. Comparing base (f09f8af) to head (bce716a).
⚠️ Report is 799 commits behind head on main.

Files with missing lines	Patch %	Lines
...n/scala/org/apache/comet/rules/CometScanRule.scala	69.04%	6 Missing and 7 partials ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #2978      +/-   ##
============================================
+ Coverage     56.12%   59.54%   +3.42%     
- Complexity      976     1377     +401     
============================================
  Files           119      167      +48     
  Lines         11743    15522    +3779     
  Branches       2251     2571     +320     
============================================
+ Hits           6591     9243    +2652     
- Misses         4012     4980     +968     
- Partials       1140     1299     +159     

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