fix(harness): drain session mirrors on close

[richuV]

Summary:

• add a deterministic FIFO barrier for SessionTree mirror work submitted before shutdown
• drain pending mirrors before HarnessAgent closes its workspace index and delegate
• replace timing-based mirror waits and cover an in-flight blocking upload

Fixes #2147

Verification: 41 focused Harness/session/memory tests passed; Spotless and git diff check passed.

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[codecov]

Codecov Report

❌ Patch coverage is 50.00000% with 7 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
...cope/harness/agent/memory/session/SessionTree.java	30.00%	6 Missing and 1 partial ⚠️

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[AgentScopeJavaBot]

🤖 AI Review

Adds deterministic FIFO barrier (awaitPendingMirrors) to SessionTree for draining pending async mirror operations before HarnessAgent.close(). Correct shutdown ordering, proper interrupt handling, good test coverage replacing unreliable Thread.sleep waits. No issues found.