fix(protoc-gen-openapiv2): prevent panic when generating OpenAPI for multiple files

[franchb]

Summary

Fixes #6274

When multiple proto files are specified as targets (e.g., via buf or protoc), the plugin would panic with "failed to resolve method FQN" because the naming cache retained a filtered mapping from a previous file that excluded method FQNs needed by the current file.

Changes

• Clear the naming cache at the start of each file's applyTemplate() so each file builds a fresh mapping with all needed names
• Add a fallback for method FQN lookup that uses FQN naming strategy instead of panicking if the lookup fails
• Add test cases for the merge scenario with body:"*" and path parameters

Root Cause

The registriesSeen cache in template.go uses *descriptor.Registry as the key. When processing multiple target files, each file's applyTemplate() overwrites the cache with its own filtered subset of names (to avoid naming collisions with unused types).

When messages.proto (with no services) is processed first:

1. The filtered mapping excludes method FQNs (since services list is empty)
2. This mapping is cached in registriesSeen[p.reg]
3. When service.proto is processed, lookups in renderServices() use the cached mapping
4. Method FQN not found → panic

Test plan

• Added TestGenerateMergeFilesWithBodyAndPathParams - tests merged files with body:"*" and path parameters
• Added TestGenerateMergeWithServiceNotInTargets - tests generating service file with messages in dependency
• All existing tests pass

🤖 Generated with Claude Code

[johanbrandhorst]

Thanks for this PR, I had a question about one of the comments.

[johanbrandhorst]

I can't find a reference for this assertion, could you explain what you mean? If this part is AI generated, please back it up with sources or remove it.

[franchb]

You're right to call this out. This test was speculative coverage I added while working on the fix, but I can't point to a specific issue it prevents. I'll remove it to keep the PR focused on the actual fix for #6274.

[johanbrandhorst]

Thanks!