Use pre-allocated static responses for fast-path bad request handling

[adityamandaleeka]

When a bad request is rejected before the app starts processing, bypass the normal response machinery (CreateResponseHeaders, header serialization, lock acquisition, WriteSuffix) and write static response fragments directly to the transport output.

The cached Date header from DateHeaderValueManager and the conditional Server header are inserted between the static prefix and suffix to maintain full HTTP compliance and response parity with the normal path.

This change adds another ~7% RPS improvement on the single-core bad request test in our perf lab. That's on top of the ~24% improvement done in #65256. The cumulative RPS improvement is ~30%.

We have pretty good test coverage already in this area and all the tests are passing.

[Copilot]

Pull request overview

This PR optimizes bad request handling in Kestrel's HTTP/1.1 implementation by introducing a fast path that uses pre-allocated static response fragments. When a request is rejected before the application starts processing, the server now bypasses the normal response machinery (header serialization, lock acquisition, WriteSuffix) and writes static response bytes directly to the transport output.

Changes:

• Changed _requestRejectedException field visibility from private to protected in HttpProtocol to allow access from derived classes
• Added pre-allocated static response fragments for common HTTP error status codes (400, 408, 414, 431, 505)
• Implemented fast-path logic in TryProduceInvalidRequestResponse() that uses static responses when appropriate
• Added GetStaticErrorResponsePrefix() method to map rejection reasons to corresponding static response prefixes

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
src/Servers/Kestrel/Core/src/Internal/Http/HttpProtocol.cs	Changed _requestRejectedException field visibility from private to protected to enable access from Http1Connection
src/Servers/Kestrel/Core/src/Internal/Http/Http1Connection.cs	Added static response fragments and fast-path implementation for early bad request handling that writes pre-allocated responses directly to transport output