Fix concurrent-safe route registration (data race in addRoute/Routes)

[mehrdadbn9]

This fixes a data race between `addRoute()` and `Routes()` functions where one goroutine appends to `trees` slice while another reads from it.

Root Cause:

• `addRoute()` reads `engine.trees` to check if method exists, then appends to the slice
• `Routes()` iterates over `engine.trees` to collect all routes
• When these run concurrently, it causes a data race

Solution:

• Added `mu sync.RWMutex` field to `Engine` struct
• `addRoute()` uses `Lock()` to ensure exclusive write access
• `Routes()` uses `RLock()` to allow concurrent reads

Testing:

• Original repro test from issue passes without race detector warnings
• Full test suite passes with `-race` flag

Related Code:

• `http.ServeMux` uses similar pattern with `sync.RWMutex` for concurrent safety

Fixes: #4457

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.27%. Comparing base (3dc1cd6) to head (a7b6562).
⚠️ Report is 246 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4524      +/-   ##
==========================================
+ Coverage   99.21%   99.27%   +0.06%     
==========================================
  Files          42       46       +4     
  Lines        3182     3042     -140     
==========================================
- Hits         3157     3020     -137     
+ Misses         17       15       -2     
+ Partials        8        7       -1     

Flag	Coverage Δ
	?
--ldflags="-checklinkname=0" -tags sonic	99.27% <100.00%> (?)
-tags go_json	99.20% <100.00%> (?)
-tags nomsgpack	99.26% <100.00%> (?)
go-1.18	?
go-1.19	?
go-1.20	?
go-1.21	?
go-1.24	99.27% <100.00%> (?)
go-1.25	99.27% <100.00%> (?)
macos-latest	99.27% <100.00%> (+0.06%)	⬆️
ubuntu-latest	99.27% <100.00%> (+0.06%)	⬆️

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[mehrdadbn9]

"Hi maintainers! 👋

All CI checks are now passing:

• ✅ Lint
• ✅ All tests (Ubuntu/macOS, Go 1.24/1.25, including race detector)
• ✅ Codecov/project
• ✅ CodeQL
• ✅ Trivy Security Scan\n\nThis PR fixes a data race between `addRoute()` and `Routes()` by adding proper mutex synchronization. The fix has been tested with the race detector enabled.\n\nWould appreciate a review when you have time. Thank you for your work on gin! 🙏"