fix(gengapic): use recursive resource name heuristic

[quartzmo]

Refactor the resource identifying field heuristic to be recursive and address specific gaps in the current implementation.

Satisfies Requirements:

1. Nested Field Support: Recursively searches for fields annotated with google.api.resource_reference up to 2 levels deep. This directly addresses AIP-134 (Update) scenarios where the resource is often nested (e.g., req.Secret.Name).
2. Tie-breaking: Prioritizes annotated fields that also appear in the method's google.api.http path template, ensuring the correct "primary" resource is selected when multiple references exist.
3. Code Generation: Returns the full field path (e.g., secret.name) to generate idiomatic Go accessors for nested fields (e.g., req.GetSecret().GetName()), addressing AIP-134 scenarios.

TODO (Remaining Requirements from Design):

• Vocabulary Learning: Implement generation-time scanning to build a
  vocabulary of valid resource types from HTTP path literals. This is
  required for legacy services (Compute, SQL, BigQuery) that lack
  annotations.
• Path Reconstruction: Implement logic to parse "exploded" HTTP
  paths and generate code to reassemble multiple request fields into a
  single canonical resource name.
• Path Pruning: Implement logic to prune non-resource segments
  (custom verbs) from reconstructed paths.
• Heuristic Allow-listing: Add configuration to restrict these
  advanced heuristics to the specific legacy services identified in the
  design.

Tests added in internal/gengapic/resource_name_test.go verify the recursive search and tie-breaking logic.

[quartzmo]

Replaced by #1722