refactor: use a consistent prefix for auto-generated names

[laststylebender14]

Summary:
Previously we were generating types names with LLM for types that already have good name (manually inferred.), this PR ensures only types with GEN__ prefix are processed with LLM.

Issue Reference(s):
Fixes #... (Replace "..." with the issue number)

Build & Testing:

• I ran cargo test successfully.
• I have run ./lint.sh --mode=fix to fix all linting issues raised by ./lint.sh --mode=check.

Checklist:

• I have added relevant unit & integration tests.
• I have updated the documentation accordingly.
• I have performed a self-review of my code.
• PR follows the naming convention of <type>(<optional scope>): <title>

[codecov]

Codecov Report

Attention: Patch coverage is 77.77778% with 16 lines in your changes missing coverage. Please review.

Project coverage is 87.17%. Comparing base (2cf0f58) to head (4b16d10).

Files with missing lines	Patch %	Lines
src/cli/llm/infer_type_name.rs	64.44%	16 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2783      +/-   ##
==========================================
- Coverage   87.18%   87.17%   -0.02%     
==========================================
  Files         256      256              
  Lines       25516    25558      +42     
==========================================
+ Hits        22247    22280      +33     
- Misses       3269     3278       +9     

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[github-actions]

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