mock: treat Anything as a matcher only in expectations

[justadityaraj]

Summary

Treat mock.Anything as a matcher only when it appears in an expected argument list.

Problem / Motivation

Arguments.Diff also treated an actual argument equal to the mock.Anything sentinel as a wildcard. As a result, a runtime call such as MethodCalled("Do", mock.Anything) could satisfy an expectation for a different literal value.

Changes

• Remove wildcard handling for mock.Anything on the actual-argument side.
• Keep the existing expectation-side matcher behavior.
• Add unit and MethodCalled regression coverage for both directions.

User impact

Mocks now reject runtime mock.Anything values that do not equal the configured expectation, preventing unexpected calls from passing silently.

Verification

• go test ./mock -count=1 — passed
• go vet ./mock — passed
• gofmt -l mock/mock.go mock/mock_test.go — clean
• go generate ./... — no generated changes
• git diff --check — passed

The full Windows run also passed the changed mock package and the unaffected root/internal/require packages. It could not complete repository-wide because existing tests require Windows symlink privileges and a race-enabled subprocess toolchain.

Related issues

Closes #1943