[QUIC] Do not allocate exception if it might not be used

[ManickaP]

Fixes #127455
Validation manual on the repro code, before:

after (no more QuicException):

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[Copilot]

Pull request overview

Reduces avoidable allocations on the QUIC stream shutdown path by preventing creation of an OperationAborted QuicException when it cannot be observed (because the stream start task source is already completed). This aligns with the perf investigation in #127455 and targets the high-frequency shutdown event path for HTTP/3 server scenarios.

Changes:

• Guard _startedTcs.TrySetException(ThrowHelper.GetOperationAbortedException()) behind _startedTcs.IsCompleted to avoid allocating an exception that would be ignored.
• Preserve existing shutdown completion semantics while reducing handled exception churn.

[rzikm]

LGTM. I assume you looked at other places where we might allocate exceptions unnecessarily?

[ManickaP]

Yep, the only other place interesting was connection shutdown. But we still need it to complete the accept queueu there, so I'm pretty sure this PR is the only relevant place.

[ManickaP]

/ba-g #127500