fix(router): make HTTPConnector and NIXLConnector stateless to prevent data races and request corruption

[bhuvan-somisetty]

What type of PR is this?

/kind bug

What this PR does / why we need it:

In HTTPConnector (pkg/kthena-router/connectors/http.go) and NIXLConnector (pkg/kthena-router/connectors/nixl.go), mutable per-request state (prefillRequest, decodeRequest, and decodeRequestBody) were maintained as struct fields on the connector instances.

When Kthena Router operates in PD disaggregation mode, connector singletons created via r.connectorFactory.GetConnector(connectorType) are shared across concurrent requests. When two or more requests are routed through HTTPConnector, NIXLConnector, or MoonCakeConnector concurrently, multiple goroutines concurrently mutate and read h.decodeRequest, h.prefillRequest, n.prefillRequest, and n.decodeRequestBody on the shared connector instance without synchronization. This causes a data race and request payload corruption, where one request's decode phase sends another concurrent request's prompt and parameters to the decode engine.

This PR makes HTTPConnector and NIXLConnector completely stateless by:

1. Removing per-request struct fields (prefillRequest, decodeRequest, decodeRequestBody) from HTTPConnector and NIXLConnector.
2. Maintaining request objects as local variables within Proxy() and passing them directly into prefill() and decode().
3. Adding concurrent regression unit tests (TestHTTPConnector_ConcurrentThreadSafety and TestNIXLConnector_ConcurrentThreadSafety) that simulate concurrent requests through a shared connector instance to verify thread safety and payload isolation.

Which issue(s) this PR fixes:

Fixes #1656

Bug evidence (required for bug-related PRs):

• Production execution path:

  1. proxyToPDDisaggregated obtains the connector singleton from factory:
     https://github.com/volcano-sh/kthena/blob/713a4a5814526d19b45781a742ea3521d9657065/pkg/kthena-router/router/router.go#L1363
  2. HTTPConnector.Proxy writes to struct fields h.decodeRequest and h.prefillRequest:
     https://github.com/volcano-sh/kthena/blob/713a4a5814526d19b45781a742ea3521d9657065/pkg/kthena-router/connectors/http.go#L74-L77
  3. NIXLConnector.Proxy writes to struct fields n.prefillRequest and n.decodeRequestBody:
     https://github.com/volcano-sh/kthena/blob/713a4a5814526d19b45781a742ea3521d9657065/pkg/kthena-router/connectors/nixl.go#L65-L67
  4. Concurrent request goroutines overwrite these shared fields prior to the decode phase dispatching decodeRequest / decodeRequestBody.

• Verified by concurrent regression unit tests running concurrent goroutines through shared connector instances.

Special notes for your reviewer:

All unit tests in pkg/kthena-router/... pass cleanly.

Does this PR introduce a user-facing change?:

NONE

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