fix: camelCase path params, double path-prefix, nested body exclusion

[george-oakling]

Summary

Three fixes for google.api.http OpenAPI generation:

• Path parameters now use camelCase (JSON names) by default instead of snake_case proto field names. For example, {account_id} becomes {accountId} in both the path template and parameter name. The with-proto-names option is respected to keep snake_case when needed.
• Fix double path-prefix for additional_bindings. The prefix was applied both inside httpRuleToPathMap (for additional bindings only) and in the central addPathItem handler, resulting in doubled prefixes like /api/node/api/node/....
• Fix nested path params not excluding the top-level field from request body. When a path parameter references a nested field (e.g., source_public_id.value), the top-level field (source_public_id) was not tracked, so it was not excluded from the body: "*" request body schema.

Changes

• googleapi/paths.go: Build a paramRenames map (proto name → camelCase display name) during path parameter resolution; pass it to partsToOpenAPIPath so URL templates also use camelCase; track top-level fields for nested path params; remove redundant util.MakePath() call on additional bindings (fixes double prefix).
• googleapi/paths_test.go: 4 new unit tests for partsToOpenAPIPath with rename map.
• New test protos: googleapi_camelcase_params.proto, googleapi_nested_body.proto, additional_bindings_prefix.proto.
• Updated golden files to reflect camelCase path params and nested field exclusion.

Rebased on current main (includes #233, #234, #239). Updated path_params golden files from #234 to reflect camelCase conversion.

Test plan

• New unit tests for partsToOpenAPIPath with rename map (4 tests)
• New integration test googleapi_camelcase_params.proto — camelCase path and query params
• New integration test googleapi_nested_body.proto — nested path param body exclusion
• New integration test scenario additional_bindings_prefix with path-prefix=/api/v1 — no double prefix
• Existing proto_names scenario confirms with-proto-names still produces snake_case
• All existing tests pass with updated snapshots

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