fix: allow proto3 optional fields in path parameters

[susanachl]

References to other Issues or PRs

Fixes #6352

Motivation & Context

This PR removes the compiler restriction (originally introduced in #1951) that prevents proto3 optional fields from being mapped to HTTP path parameters.

Addressing the Routing Ambiguity Concern:
By removing this check, we decouple the Protobuf schema validation from the HTTP routing layer. If an optional field is mapped to a path parameter like /owners/{owner_name}/pets/, the structural requirement of the URL template remains intact.

If a client omits the optional field, resulting in an empty path segment (POST /v1/owners//pets/fido:feed), the underlying gRPC-Gateway HTTP router will naturally fail to match the template and return a 404 Not Found. This correctly shifts the responsibility of validating path completeness from the proto compiler to the HTTP router, which is the expected behavior for malformed requests.

Changes

• Removed proto3_optional check in internal/descriptor/services.go.
• Refactored TestOptionalProto3URLPathMappingError to TestOptionalProto3URLPathMappingSuccess in internal/descriptor/services_test.go.
• Added an additional_binding to the Custom RPC in examples/internal/proto/examplepb/a_bit_of_everything.proto using an optional path parameter to prove end-to-end generation success.

[johanbrandhorst]

LGTM, thank you! I can't think of a reason this would break any existing users, and since this is something multiple users want, lets try it and see how it goes :).

[johanbrandhorst]

Looks like you need to run the regenerate command, see CONTRIBUTING.md for instructions. Thanks!