Resolving always-false conditional making some compilers miss for preprocessor checks

[Joldiges]

Standard #line directives were never recognized while collecting included
files for preprocessor-cache manifests because the parser compared four bytes
with the five-byte line prefix.

The existing line
|| (&slice[1..5] == b"line "))
Was comparing 4 bytes against 5 causing this conditional to never hit.

This change was primarily made to fix Tasking Compiler runs.

GitHub copilot was used for the debugging and resolution of this issue.

[Copilot]

Pull request overview

Fixes preprocessor-cache manifest collection for C/C++ preprocessed output by correctly recognizing standard #line directives (previously the condition was always false due to a slice-length mismatch), which could cause cache misses for some compilers (notably Tasking).

Changes:

• Correct #line directive detection in process_preprocessed_file by switching to a proper prefix check (starts_with(b"line ")).
• Add a unit test validating that #line directives cause the referenced header to be recorded and hashed in included_files.

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[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 73.74%. Comparing base (8ab3926) to head (adbe9b8).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2824      +/-   ##
==========================================
+ Coverage   73.71%   73.74%   +0.02%     
==========================================
  Files          72       72              
  Lines       37932    37975      +43     
==========================================
+ Hits        27963    28006      +43     
  Misses       9969     9969              

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