feat(legacylibrarian): add preview versioning semantics

[noahdietz]

Adds preview versioning semantics to legacylibrarian release stage. When running with -branch=preview, the version of the client on the main branch i.e. the "stable version" is consulted as described in #2764.

Updated just the most "code local" test. Local integration testing on a fork also indicates it works.

Note: This does do a github lookup of the state.yaml on a per library basis. Ideally this code is only used for the one API we will ship previews for in the short term and so optimizations i.e. caching were not implemented.

Fixes #3647

[gemini-code-assist]

Summary of Changes

Hello @noahdietz, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request introduces robust preview versioning semantics to the legacylibrarian release stage process. Its primary goal is to ensure that when a release is staged from a preview branch, the versioning correctly accounts for the stable version on the main branch, providing a more consistent and predictable versioning scheme for pre-releases. This enhancement improves the accuracy of version derivation for preview builds.

Highlights

• Preview Versioning Semantics: Implemented new logic for legacylibrarian release stage to handle preview versions, specifically when running on a -branch=preview.
• Stable Version Consultation: The system now consults the "stable version" from the main branch to correctly derive the next preview version, addressing the issue described in librarian: implement stable-relative versioning for preview track #2764.
• Context Propagation: The context.Context parameter has been added to processLibrary, updateLibrary, and determineNextVersion functions for better cancellation and request-scoped value handling.
• New Test Cases: Added comprehensive test cases for determineNextVersion to validate the new preview versioning logic, including scenarios for prerelease bumps.

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[gemini-code-assist]

Code Review

This pull request introduces preview versioning semantics, which is a great feature. The implementation correctly fetches the stable version from the main branch to derive the next preview version, and the context propagation through the call stack is a good improvement.

I've left a few comments:

• A high-severity suggestion to improve performance by caching the main branch state to avoid redundant API calls.
• A medium-severity suggestion to clarify a comment in loadStableLibraryVersion.
• A medium-severity suggestion for a more idiomatic variable declaration.

The test cases for the new preview logic are well-written and cover important scenarios. Overall, this is a solid implementation.

[codecov]

Codecov Report

❌ Patch coverage is 85.18519% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 82.42%. Comparing base (737e0d3) to head (efbdae6).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
...l/legacylibrarian/legacylibrarian/release_stage.go	85.18%	2 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3686      +/-   ##
==========================================
- Coverage   82.43%   82.42%   -0.01%     
==========================================
  Files         138      138              
  Lines       12657    12674      +17     
==========================================
+ Hits        10434    10447      +13     
- Misses       1733     1735       +2     
- Partials      490      492       +2     

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