Adapt the response-ownership guard from e905b5b to the v6 handler interface. Abort terminal attempts instead of replacing a response already exposed downstream, while preserving initial response forwarding.

Add handler-level regression coverage for terminal errors and initial non-retryable responses.

* fix: complete upgrade diagnostics lifecycle

Successful CONNECT and protocol upgrades stop before Undici marks the request complete. Diagnostics subscribers retain request state, and streamed request-body listeners remain attached to the upgraded socket.

Publish the existing response lifecycle around accepted upgrades. Preserve the early HTTP/2 CONNECT handoff in v6, and terminate diagnostics if the stream fails before its response.

Refs: #5783
Signed-off-by: Ruben Bridgewater <ruben.bridgewater@datadoghq.com>

* fix: preserve abort after upgrade handoff

Completing upgrade requests suppresses late request errors, but v6 retains the abort callback as the transport cleanup owner after handoff. Returning early leaves upgraded sockets and HTTP/2 CONNECT streams open.

Signed-off-by: Ruben Bridgewater <ruben.bridgewater@datadoghq.com>

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Signed-off-by: Ruben Bridgewater <ruben.bridgewater@datadoghq.com>

The test races its forced body timeout against both the first response chunk and a native response-completion timer. A byte-zero retry can succeed, while an early response completion can prevent the expected rejection.

An open RetryAgent can also keep server teardown pending after the assertion completes.

Signed-off-by: Ruben Bridgewater <ruben.bridgewater@datadoghq.com>

Co-authored-by: github-actions <github-actions@github.com>