refactor(headers): centralize custom HTTP headers into dedicated package

[Xunzhuo]

• Create new pkg/headers package to manage all x-* prefixed headers
• Define constants for request, VSR decision tracking, and security headers
• Update security headers to use x-vsr-* prefix for consistency
  • x-pii-violation -> x-vsr-pii-violation
  • x-jailbreak-blocked -> x-vsr-jailbreak-blocked
  • x-jailbreak-type -> x-vsr-jailbreak-type
  • x-jailbreak-confidence -> x-vsr-jailbreak-confidence
• Replace all string literals with constants across codebase
• Fix variable naming conflict in request_handler.go

BREAKING CHANGE: Security header names changed from x-* to x-vsr-* prefix. Clients consuming these headers must update to use new names.

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[github-actions]

👥 vLLM Semantic Team Notification

The following members have been identified for the changed files in this PR and have been automatically assigned:

📁 src

Owners: @rootfs, @Xunzhuo, @wangchen615
Files changed:

• src/semantic-router/pkg/headers/headers.go
• src/semantic-router/pkg/headers/headers_test.go
• src/semantic-router/pkg/extproc/request_handler.go
• src/semantic-router/pkg/extproc/response_handler.go
• src/semantic-router/pkg/utils/http/response.go

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🎉 Thanks for your contributions!

This comment was automatically generated based on the OWNER files in the repository.