Fix RequestReply duplicate reply deadlock

[rootp1]

Fixes #9190

Proposed Changes

• stop sending duplicate reply events while holding the RequestReply inflight-map read lock
• make duplicate reply delivery non-blocking so cleanup can still acquire the write lock
• add a regression test that would previously hang on a duplicate reply hitting a full reply channel

Pre-review Checklist

• At least 80% unit test coverage
  Current local run: go test -cover ./pkg/requestreply reported 77.1% statement coverage for pkg/requestreply. This PR adds a focused regression test, but does not yet bring the package above the template target on its own.
• E2E tests for any new behavior
  Not applicable for this change as implemented in this PR. The fix is confined to the in-process RequestReply handler concurrency path, and the regression is covered with a focused unit test in pkg/requestreply/ingress_handler_test.go.
• Docs PR for any user-facing impact
  Not applicable. This is an internal reliability fix with no user-facing API, CLI, or workflow change.
• Spec PR for any new API feature
  Not applicable. No API surface or behavior contract is being extended.
• Conformance test for any change to the spec
  Not applicable. There is no spec change in this PR.

Release Note

Fixed a RequestReply deadlock that could occur when duplicate reply events were delivered for the same inflight request.

Docs

No docs update required; this is an internal reliability fix with no API or user workflow change.

[knative-prow]

Welcome @rootp1! It looks like this is your first PR to knative/eventing 🎉

[knative-prow]

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[knative-prow]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: rootp1
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[rootp1]

@Cali0707, please have a look, whenever you have a chance to