impl(v3): remove forwarding headers

[scotthart]

No description provided.

[gemini-code-assist]

Summary of Changes

Hello @scotthart, 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 focuses on cleaning up and modernizing the client library by eliminating deprecated forwarding headers. This change simplifies the overall architecture, reduces maintenance overhead, and encourages direct usage of versioned API headers, leading to a more explicit and robust dependency structure for client applications.

Highlights

• Removal of Forwarding Headers: The pull request completely removes all unversioned forwarding headers and their associated generation logic from the codebase. This includes client, connection, idempotency policy, and options headers.
• Build System Simplification: The build configurations (CMakeLists.txt, BUILD.bazel) and generator logic have been updated to no longer create or reference these deprecated forwarding headers, streamlining the build process.
• Migration Guide Update: The v3-migration-guide.md documentation has been updated to inform users about the removal of these headers and provides clear instructions on how to update their include paths to use the versioned headers directly.

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[gemini-code-assist]

Code Review

This pull request is a large-scale refactoring to remove deprecated forwarding headers. The changes are extensive, touching generator logic, build configurations, documentation, and removing a significant number of header files. This is a valuable cleanup that improves the maintainability of the codebase by removing a layer of backward compatibility. The changes appear to be systematic and correct. I have one suggestion to improve the completeness of the migration guide.

[gemini-code-assist]

The list of libraries with removed forwarding headers appears to be missing google/cloud/cloudquotas. The changes in google/cloud/cloudquotas/CMakeLists.txt indicate that it also had forwarding headers removed in this PR. For completeness of this migration guide, please consider adding it to the list.

[scotthart]

cloudquotas does not have forwarding headers, but it did have an unnecessary service dir in its CMakeLists.txt file.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.87%. Comparing base (44b5475) to head (9ed3105).

Additional details and impacted files

@@                  Coverage Diff                   @@
##           prepare-for-v3.0.0   #15907      +/-   ##
======================================================
+ Coverage               92.83%   92.87%   +0.04%     
======================================================
  Files                    2354     2343      -11     
  Lines                  216298   216117     -181     
======================================================
- Hits                   200794   200719      -75     
+ Misses                  15504    15398     -106     

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