feat(engine): add Shutdown method for graceful shutdown

[Narita-1095305]

Add Engine.Shutdown() method for graceful shutdown support

Description

This PR adds built-in graceful shutdown support to Gin Engine, allowing users to properly wait for active connections to finish before shutting down the server.

Changes

• Add server and serverLock fields to Engine struct to store server reference
• Add Shutdown(ctx context.Context) method that calls http.Server.Shutdown()
• Modify Run(), RunTLS(), RunUnix(), RunListener() to store server reference
• Add RunWithShutdown() convenience method with built-in signal handling
• Add comprehensive tests for graceful shutdown

Usage

router := gin.Default()
router.GET("/", handler)

go router.Run(":8080")

// Wait for signal
quit := make(chan os.Signal, 1)
signal.Notify(quit, syscall.SIGINT, syscall.SIGTERM)
<-quit

// Graceful shutdown
ctx, cancel := context.WithTimeout(context.Background(), 10*time.Second)
defer cancel()
router.Shutdown(ctx)

Or use the convenience method:

router := gin.Default()
router.GET("/", handler)
router.RunWithShutdown(":8080", 10*time.Second) // Handles SIGINT/SIGTERM automatically

Related Issue

Addresses the need for built-in graceful shutdown support mentioned in documentation.

Checklist

• I have read the contributing guidelines
• I have added tests that prove my fix is effective or that my feature works
• I have commented my code, particularly in hard-to-understand areas

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.01%. Comparing base (3dc1cd6) to head (f894f4c).
⚠️ Report is 232 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4501      +/-   ##
==========================================
- Coverage   99.21%   99.01%   -0.21%     
==========================================
  Files          42       45       +3     
  Lines        3182     3035     -147     
==========================================
- Hits         3157     3005     -152     
- Misses         17       21       +4     
- Partials        8        9       +1     

Flag	Coverage Δ
	?
--ldflags="-checklinkname=0" -tags sonic	99.00% <100.00%> (?)
-tags go_json	98.93% <100.00%> (?)
-tags nomsgpack	98.99% <100.00%> (?)
go-1.18	?
go-1.19	?
go-1.20	?
go-1.21	?
go-1.24	99.01% <100.00%> (?)
go-1.25	99.01% <100.00%> (?)
macos-latest	99.01% <100.00%> (-0.21%)	⬇️
ubuntu-latest	99.01% <100.00%> (-0.21%)	⬇️

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[Narita-1095305]

Note on Codecov Project Coverage

The codecov/project check is failing due to a -0.21% decrease in overall project coverage (99.22% → 99.01%).

Verification

• ✅ Patch coverage: 100% - All newly added code is fully covered by tests
• ✅ Shutdown(): 100% coverage
• ✅ RunWithShutdown(): 100% coverage
• ✅ RunWithShutdownConfig(): 100% coverage

Cause of Coverage Drop

The slight decrease is not caused by the code in this PR. Local testing confirms:

github.com/gin-gonic/gin/gin.go:555:       Shutdown                  100.0%
github.com/gin-gonic/gin/graceful.go:31:   RunWithShutdown           100.0%
github.com/gin-gonic/gin/graceful.go:40:   RunWithShutdownConfig     100.0%

The project-wide coverage drop is related to existing packages (e.g., codec/json: 0%, ginS: 73.1%) rather than this PR's changes.

I believe this PR should be acceptable for review since all modified and added code has 100% test coverage.