Extract shared single-flight teardown helper

[Copilot]

Server-mode client types duplicated the lock-and-cache pattern used to coordinate concurrent teardown requests. This made the idempotency guarantee easy to change inconsistently.

• Shared teardown coordination

  • Add SingleFlightTask to schedule an action once and return the same task to every caller.
  • Support synchronous and asynchronous teardown delegates.

• Client adoption

  • Replace duplicated shutdown locks and cached tasks in MtpServerClient, MtpServerProcess, and MtpServerInProcessHost.
  • Keep the in-process host’s read-loop flag update outside the shared helper.

• Coverage

  • Add concurrent-call tests covering task identity and exactly-once execution for both delegate forms.

private readonly SingleFlightTask _shutdown = new();

private Task StartShutdownAsync()
    => _shutdown.StartAsync(ShutdownCore);

• Fixes [duplicate-code] Duplicate Code: Single-Flight Teardown Pattern Repeated in ServerMode.Client Types #11481

[Copilot]

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[Copilot]

Copilot review overview

🟡 Changes recommended

The new test uses non-generic TaskCompletionSource, which prevents the Windows net462 target from compiling.

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Review effort: Balanced
Findings: 1

Open (1)

• Use generic TaskCompletionSource for net462 compatibility · New

[Copilot]

Copilot review overview

🟢 Approval recommended

The refactor preserves existing teardown semantics and is adequately covered across both delegate forms.

Review effort: Balanced
Findings: None

Resolved since last review (1)

• Use generic TaskCompletionSource for net462 compatibility