fix(build): skip rerun-if-changed for .files() in Buf mode

[hobostay]

Summary

• Skip emitting cargo:rerun-if-changed for .files() entries when using Config::use_buf(), since those entries are proto-relative names (not filesystem paths)

Problem

In Buf mode, .files() holds proto-relative names like "my/service.proto", not filesystem paths. Emitting cargo:rerun-if-changed=my/service.proto points cargo at a non-existent file, which forces a rebuild on every invocation.

This is the same issue that was fixed for Precompiled mode in #56, but Buf mode was missed in that fix. The comment on DescriptorSource::Precompiled already explains the rationale:

self.files holds proto-relative names (per the docs on descriptor_set()), not on-disk paths; emitting rerun-if-changed for them points cargo at missing files and forces a rebuild on every invocation.

The same applies to Buf mode — the docs on Config::use_buf() say:

Config::files() must contain proto-relative names as they appear in the buf module (e.g. "my/service.proto"), not filesystem paths.

Test plan

• Existing tests pass (cargo test -p connectrpc-build)
• The fix mirrors the approach already used for Precompiled mode

🤖 Generated with Claude Code

[github-actions]

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[iainmcgin]

@hobostay you need to post "I have read the CLA Document and I hereby sign the CLA" here too - I know you did this for the buffa repo, but the CLA records are separately managed in each repo.