[test-improver] Improve tests for httputil package (endpoint helpers)

[github-actions]

Test Improvements: httputil_test.go

File Analyzed

• Test File: internal/httputil/httputil_test.go
• Package: internal/httputil
• Lines of Code: 427 → 484 (+57 lines)

Improvements Made

1. Increased Coverage

• ✅ Added TestWriteSimpleHealthResponse — covers WriteSimpleHealthResponse (was 0%)
• ✅ Added TestWriteReflectResponse with 3 subtests — covers WriteReflectResponse (was 0%)
• Previous Coverage: 96.6%
• New Coverage: 100.0%
• Improvement: +3.4%

2. Better Testing Patterns

• ✅ Used t.Run() subtests for the reflect response scenarios
• ✅ Used require.NoError before assertions that depend on successful decode
• ✅ Descriptive subtest names that document expected behaviour
• ✅ RFC3339 timestamp validation via time.Parse

3. Coverage Scenarios Added

Subtest	What it verifies
TestWriteSimpleHealthResponse	Status 200, application/json Content-Type, {"status":"ok"} body with exactly one field
TestWriteReflectResponse/empty_components_returns_valid_response_shape	Non-nil Agents map and non-empty Timestamp for a zero-value DIFCComponents
TestWriteReflectResponse/strict_mode_is_reflected_in_response_body	mode:"strict" propagated from difc.NewComponents through the response
TestWriteReflectResponse/timestamp_is_an_RFC3339_string	Timestamp field parses as a valid RFC3339 string

Test Execution

All tests pass:

=== RUN   TestWriteSimpleHealthResponse
--- PASS: TestWriteSimpleHealthResponse (0.00s)
=== RUN   TestWriteReflectResponse
=== RUN   TestWriteReflectResponse/empty_components_returns_valid_response_shape
=== RUN   TestWriteReflectResponse/strict_mode_is_reflected_in_response_body
=== RUN   TestWriteReflectResponse/timestamp_is_an_RFC3339_string
--- PASS: TestWriteReflectResponse (0.00s)
PASS
coverage: 100.0% of statements
ok  github.com/github/gh-aw-mcpg/internal/httputil  0.008s  coverage: 100.0% of statements

Why These Changes?

WriteSimpleHealthResponse and WriteReflectResponse in endpoint_helpers.go are shared helpers used by both the server and proxy packages to ensure consistent health and DIFC reflect endpoint responses. Despite being production-critical shared utilities, they had 0% test coverage. The new tests confirm correct HTTP status codes, Content-Type headers, response body shapes, and DIFC mode/timestamp fields.

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