dotnet: Deflake warm pending-tool cleanup

[devm33]

Problem

The .NET SDK Tests validation run 35724488486 hung for ten minutes in Should_Keep_Pending_External_Tool_Handleable_On_Warm_Resume_When_ContinuePendingWork_Is_False.

The test intentionally leaves the original external-tool callback blocked while a second client resumes the active session and completes that pending request. After verifying that the external completion succeeded, it disposed the resumed session before releasing the original callback. Cleanup could therefore wait indefinitely for the callback whose release lived in the later finally block.

The test is worthwhile because it verifies that an active session resumed with ContinuePendingWork = false still exposes its pending external tool request for explicit completion.

Changes

Release the original blocked callback immediately after the resumed client successfully handles the pending tool call and verifies the single invocation. The existing finally remains as failure-path cleanup.

Product impact

No product behavior changes. This only fixes test cleanup ordering after the behavior under test has been verified.

Validation

• git diff --check
• Ten exact-head .NET SDK Tests workflow runs pending

[Copilot]

Copilot review overview

🟢 Approval recommended

The focused cleanup-order change resolves the documented hang without altering product behavior.

Review effort: Balanced
Findings: None

What changed in this PR

Deflakes .NET E2E cleanup by releasing the blocked original tool callback before disposing the resumed session.

Changes:

• Releases the callback after successful pending-tool verification.
• Retains finally cleanup for failure paths.

File	Description
dotnet/​test/​E2E/​PendingWorkResumeE2ETests.cs	Prevents resumed-session disposal from waiting indefinitely.

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