feat: add support for custom fern.summary proto option

[devin-ai-integration]

feat: add support for custom fern.summary proto option

Summary

This PR adds support for a custom protobuf option fern.summary that allows users to explicitly set the display name for gRPC endpoints in their proto files. The custom summary is emitted as the OpenAPI operation's summary field, which Fern CLI uses as the endpoint display name.

Usage example:

import "fern/options.proto";

rpc CreateComment(CreateCommentRequest) returns (CreateCommentResponse) {
  option (google.api.http) = { post: "/comments" };
  option (fern.summary) = "Add a Comment";  // Custom display name
}

Changes:

• Added extensions/fern/options.proto defining the custom option (field number 51001)
• Generated Go code from the proto definition
• Modified generator.go to read the fern.summary option and set it on the OpenAPI operation's Summary field
• The option is read after building the operation but before merging generic Operation annotations

Review & Testing Checklist for Human

• Verify option ordering is correct: The code reads fern.summary BEFORE merging the generic Operation annotation (line 765-770 vs 772-776). This means if both are set, the Operation annotation will override fern.summary. Is this the intended behavior, or should fern.summary take precedence?

• Confirm field number 51001 is appropriate: Custom protobuf extensions should use field numbers in specific ranges. Verify that 51001 is in the correct range and doesn't conflict with other extensions.

• Test end-to-end: While this was tested through the Fern CLI test suite, please test manually with a real proto file to ensure:

  • The option is correctly read from proto files
  • The summary appears in the generated OpenAPI YAML
  • The option works with buf dependencies (users need to import the proto file)

• Consider adding unit tests: This PR has no tests in the protoc-gen-openapi repo itself. Should we add tests for the new functionality?

Notes

• This PR is part of a larger feature to support custom endpoint display names in gRPC. The corresponding Fern CLI changes are in PR #10552.
• The type assertion on line 767 (extFernSummary.(string)) is safe because protobuf extensions are strongly typed, but we could add better error handling if needed.
• No validation is performed on the summary string (length, special characters, etc.). This matches the behavior of other OpenAPI fields.

Devin session: https://app.devin.ai/sessions/35ba7fe604104e64afe342a676df4463
Requested by: kenny@buildwithfern.com

[devin-ai-integration]

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