fix: preserve user headers in cat_file requests

[fallenmi]

Fixes #889.

Summary

• Preserve caller-provided headers in _cat_file requests.
• Let explicit start/end bounds replace any case-insensitive caller Range without mutating the caller's mapping.
• Remove the temporary sequential fallback from Introduce async API in Prefetcher and Integrate it with Disk Reads #877 so concurrent get_file downloads retain conditional headers too.

Testing

• On the upstream base, the three new regression cases failed: both _cat_file header cases dropped If-Match, and headers forced get_file onto the sequential path.
• After rebasing onto current main (ab6bc825), six focused header, routing, and adjacent generation tests passed with STORAGE_EMULATOR_HOST=http://localhost:4443.
• A two-chunk concurrent probe preserved If-Match, generated the correct range per chunk, and left the caller mapping unchanged.
• flake8, isort --check-only, Black, and git diff --check passed.

The full emulator-backed suite was not run locally because the Docker daemon is unavailable; upstream CI remains the integration gate.

AI assistance disclosure

OpenAI Codex prepared the issue reproduction, implementation, tests, rebase, and validation. The GitHub account owner explicitly authorized publication and transition to ready for maintainer review.

[gemini-code-assist]

Code Review

This pull request enables passing custom headers to sequential and concurrent file operations in gcsfs. Specifically, _cat_file_sequential now correctly merges and overrides range headers when start/end limits are provided, and _get_file no longer bypasses the concurrent download path when custom headers are present. Corresponding unit tests have been added to verify these behaviors. I have no additional feedback to provide as the changes are well-implemented and covered by tests.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.07%. Comparing base (ab6bc82) to head (9222b47).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1013      +/-   ##
==========================================
+ Coverage   90.04%   90.07%   +0.03%     
==========================================
  Files          16       16              
  Lines        3638     3639       +1     
==========================================
+ Hits         3276     3278       +2     
+ Misses        362      361       -1     

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