feat(test): add RunTestHandler to flush status in test contexts

[abhiyankhanal]

Summary

• Adds RunTestHandler(w, req, handlers...) helper function to test_helpers.go
• Solves the common issue where ctx.Status() doesn't flush the status code to httptest.ResponseRecorder.Code when using CreateTestContext directly
• CreateTestContext behavior is unchanged (backward compatible)

Related Issue

Closes #3443

Changes Made

• test_helpers.go: Added RunTestHandler function that creates a test context, runs handlers, and calls WriteHeaderNow() to flush the status
• test_helpers_test.go: Added 5 test cases covering status flushing, middleware chains, default status, request setting, and backward compatibility

Testing

• go test -run "TestRunTestHandler|TestCreateTestContextBackward" -v -count=1 — all tests pass

Example Usage

// Before (broken - w.Code stays 200):
w := httptest.NewRecorder()
ctx, _ := gin.CreateTestContext(w)
ctx.Request = req
MyHandler(ctx)
assert.Equal(t, 201, w.Code) // FAILS

// After (works):
w := httptest.NewRecorder()
req := httptest.NewRequest("POST", "/", nil)
ctx := gin.RunTestHandler(w, req, MyHandler)
assert.Equal(t, 201, w.Code) // PASSES

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.31%. Comparing base (3dc1cd6) to head (3f607b8).
⚠️ Report is 268 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4570      +/-   ##
==========================================
- Coverage   99.21%   98.31%   -0.90%     
==========================================
  Files          42       48       +6     
  Lines        3182     3148      -34     
==========================================
- Hits         3157     3095      -62     
- Misses         17       44      +27     
- Partials        8        9       +1     

Flag	Coverage Δ
	?
--ldflags="-checklinkname=0" -tags sonic	98.30% <100.00%> (?)
-tags go_json	98.23% <100.00%> (?)
-tags nomsgpack	98.29% <100.00%> (?)
go-1.18	?
go-1.19	?
go-1.20	?
go-1.21	?
go-1.25	98.31% <100.00%> (?)
go-1.26	98.31% <100.00%> (?)
macos-latest	98.31% <100.00%> (-0.90%)	⬇️
ubuntu-latest	98.31% <100.00%> (-0.90%)	⬇️

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