fix(static): preserve matched handler 404s

[JSap0914]

Fixes #2775

Preserves 404 responses from matched handlers when HTML5 static fallback is enabled, while retaining the index fallback for router misses, including group route-not-found handling.

Verification:

• go test -race ./middleware -run '^TestStaticHTML5PreservesMatchedHandlerNotFound$' -count=1
• go test -race ./middleware -run '^TestStatic$/^ok,_when_html5_mode_serve_index_for_any_static_file_that_does_not_exist$' -count=1
• go test -race ./middleware

[aldas]

please separate these 2 'test cases' by table tests of something like that.

[JSap0914]

Refactored the two test cases into a table-driven test in middleware/static_test.go as requested. go test ./middleware/ -run TestStatic passes.