[test-improver] Improve tests for resolve_guard_policy_test.go

[github-actions]

Summary

Refactors internal/server/resolve_guard_policy_test.go to reduce duplication and improve structure, per the Test Improver workflow.

File analyzed: internal/server/resolve_guard_policy_test.go (505 lines, 32 standalone Test* functions, no table-driven tests, 0 manual error-check anti-patterns — testify was already used throughout).

Improvements made

• Converted the 10 TestNormalizeScopeKind_* functions into a single table-driven TestNormalizeScopeKind with t.Run subtests (kept TestNormalizeScopeKind_DoesNotMutateInput separate since it asserts on the input map, not just the result, which doesn't fit the table cleanly).
• Converted the 18 TestResolveGuardPolicy_* functions into a single table-driven TestResolveGuardPolicy, using an optional wantPolicyCheck callback for cases needing to inspect the parsed policy contents (vs. simple identity comparison for pass-through cases).
• Converted the 5 TestResolveWriteSinkPolicy_* functions into a single table-driven TestResolveWriteSinkPolicy.
• Preserved every original test case, assertion, and comment/rationale (e.g. why empty GuardPolicySource defaults to "override", why an empty policy struct is invalid, legacy-fallback semantics, etc.) — no test coverage was dropped.
• No production code changes; this is a test-only refactor.

Coverage before/after

Coverage of the exercised functions was already 100% and remains 100% after the refactor (verified via go tool cover -func):

github.com/github/gh-aw-mcpg/internal/server/guard_init.go:314  resolveGuardPolicyForAgent  100.0%
github.com/github/gh-aw-mcpg/internal/server/guard_init.go:328  resolveGuardPolicy          100.0%
github.com/github/gh-aw-mcpg/internal/server/guard_init.go:380  resolveWriteSinkPolicy      100.0%

Overall internal/server package coverage unchanged at 95.2%.

Test output

$ go test -v ./internal/server/ -run "TestNormalizeScopeKind|TestResolveGuardPolicy|TestResolveWriteSinkPolicy"
...
PASS
ok  	github.com/github/gh-aw-mcpg/internal/server	0.017s

Also verified:

• go test -count=3 ./internal/server/... — passes (no flakiness)
• go vet ./... — clean
• gofmt -l internal/server/resolve_guard_policy_test.go — no output (already formatted)
• make build and make lint — pass (golangci-lint binary unavailable in this sandbox; gofmt check passed)

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