fix: isolate request ID header configuration

[Gav1nnn]

Summary

• remove the package-level mutable request ID header configuration
• keep each middleware instance header key isolated and make it available to Get through the current Gin context
• add regression coverage for independent custom header keys and concurrent New calls

Testing

• go test ./...
• go test -race -count=10 ./...
• go vet ./...
• golangci-lint v2.12.0

Fixes #70