Feature/get request body reuse

[ljluestc]

Implement body caching for form-urlencoded bindings so the request body can be read multiple times.

1. Cache-aware GetRawData()

Modified GetRawData in context.go to check for cached body bytes in the context (BodyBytesKey) before reading from the stream.

2. Body preservation in ShouldBindWith()

Modified ShouldBindWith in context.go to intercept form and form-urlencoded bindings:

1. Read request body into memory.
2. Cache body bytes in context (BodyBytesKey).
3. Restore Request.Body with a fresh io.NopCloser buffer.
4. Proceed with binding.

This ensures ParseForm works on the restored body, and GetRawData can retrieve the cached body.

3. Test Fixes

• New Regression Test: Added TestContextGetRawDataAfterShouldBind to verify the fix.
• Fixed TestRunEmpty: Updated this integration test to use a dynamic free port instead of hardcoded :8080, resolving local testing conflicts.

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Testing

All tests pass:

$ go test ./...
ok      github.com/gin-gonic/gin                0.247s
ok      github.com/gin-gonic/gin/binding        (cached)
ok      github.com/gin-gonic/gin/render         (cached)