feat: add warning for multiple response writes

[xingzihai]

Summary

Adds a warning when attempting to write a response body multiple times. This helps developers debug issues where response bodies are written more than once.

Problem

When calling response write methods multiple times (e.g., c.Render() followed by another write), the behavior can be confusing. Developers may not realize they're overwriting a previous response.

Solution

• Added a check in Context.Render() to detect if response body was already written
• When detected, prints a warning using debugPrint in debug mode
• No behavior change in release mode

Changes

• context.go (+4 lines) - Added multiple write detection
• context_test.go (+44 lines) - Added tests for debug and release modes

Testing

• Added TestRenderMultipleWritesWarning - verifies warning in debug mode
• Added TestRenderMultipleWritesNoWarningInReleaseMode - verifies no warning in release mode

Fixes #4477

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.43%. Comparing base (3dc1cd6) to head (ec2c838).
⚠️ Report is 275 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4606      +/-   ##
==========================================
- Coverage   99.21%   98.43%   -0.78%     
==========================================
  Files          42       48       +6     
  Lines        3182     3140      -42     
==========================================
- Hits         3157     3091      -66     
- Misses         17       40      +23     
- Partials        8        9       +1     

Flag	Coverage Δ
	?
--ldflags="-checklinkname=0" -tags sonic	98.42% <100.00%> (?)
-tags go_json	98.29% <100.00%> (?)
-tags nomsgpack	98.35% <100.00%> (?)
go-1.18	?
go-1.19	?
go-1.20	?
go-1.21	?
go-1.25	98.43% <100.00%> (?)
go-1.26	98.37% <100.00%> (?)
macos-latest	98.37% <100.00%> (-0.84%)	⬇️
ubuntu-latest	98.43% <100.00%> (-0.78%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.