proto_api_scrubber: return 404 instead of 403 for blocked methods.

[sumitkmr2]

Description: Return 404 instead of 403 for blocked methods

Additional Description:
This change updates the proto_api_scrubber filter to return 404 Not Found (gRPC status 5 NOT_FOUND) instead of 403 Forbidden (gRPC status 7 PERMISSION_DENIED) when a request is rejected due to a method-level blocking rule.

Security Motivation:
The primary driver for this change is to prevent potential attackers from enumerating valid endpoints. By returning 403, the server explicitly signals that the method exists but the client is not authorized to access it. Switching to 404 ensures that clients cannot distinguish between a method that does not exist and a method that they are blocked from accessing. This reduces information leakage regarding the API surface area.

Changes:

• Updated proto_api_scrubber/filter.cc to reject requests with Status::NotFound when a method-level match occurs.
• Updated proto_api_scrubber_test/filter_test.cc unit tests to verify the new status code and local reply string.
• Updated proto_api_scrubber_test/integration_test.cc to verify the end-to-end behavior returns gRPC status 5.

Risk Level:
Low (Behavior change for blocked requests).

Testing:

• Verified via updated unit tests (MethodLevelRestrictionTest).
• Verified via updated integration tests (RejectsBlockedMethod).

[repokitteh-read-only]

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Caused by: #42885 was opened by sumitkmr2.

see: more, trace.