test(protoresolve): implement missing logic in testResolver helper

[Aaditya-ops-cpu]

This PR addresses the // TODO in resolver_test.go by implementing the verification logic for testResolver.

Changes:

1. Implemented testResolver:

   • Added logic to verify that FindFileByPath correctly returns the expected file (desc_test1.proto).
   • Added a negative test case to ensure it returns an error for non-existent files.

2. Updated registry_test.go:

   • TestRegistry and TestFromFiles previously passed empty registries to the helper.
   • Since testResolver now strictly checks for desc_test1.proto, I updated these tests to register the file before calling the helper, ensuring they pass.

Verification:-

• Ran make test locally , all tests passed, including the new logic.
• Ran make ci locally which failed at the errcheck step due to a local toolchain issue (internal error: package ... without types), but the code generation and tests are clean. Relying on GitHub Actions for the final lint check.

Verification Screenshot: -

[Aaditya-ops-cpu]

Note: This PR builds upon previous PR (which sets up the test files). I merged those changes here to verify the tests pass locally.