refactor(sidekick/rust): unify resource name template generation

[haphungw]

The Rust generator previously handled explicit fields with ResourceNameFields and heuristic templates with ResourceNameTemplate differently, creating overly complex branch logic. Both explicit resource reference annotations and path-based heuristics are now unified into a single generation source with TargetResource.

This eliminates redundant structs and logic for the codebase, while ensuring that resource names are consistently routed for both heuristic and explicit logic when detailed tracing attributes are enabled.

Fixes #4537

[gemini-code-assist]

Code Review

This pull request refactors resource identification logic, primarily by inverting the priority of heuristic over explicit identification in resource_identification.go. A significant change in internal/sidekick/rust/annotate.go involves the complete removal of the resourceNameCandidateField struct and related functions for identifying and annotating resource name fields within methodAnnotation, along with the ResourceNameFields and HasResourceNameFields fields themselves. While a new HasResourceNameArgs field was added to pathBindingAnnotation for template simplification, a critical issue was introduced: the grpc-client/transport.rs.mustache template now attempts to use the removed Codec.HasResourceNameFields and Codec.ResourceNameFields for detailed tracing attributes, which will lead to code generation failures or incorrect output due to missing data.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 79.74%. Comparing base (19f0fd4) to head (5a6be0e).
⚠️ Report is 24 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4538   +/-   ##
=======================================
  Coverage   79.74%   79.74%           
=======================================
  Files         109      109           
  Lines        9118     9118           
=======================================
  Hits         7271     7271           
  Misses       1339     1339           
  Partials      508      508           

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[haphungw]

already included in #4560