fix(platform-ios): gracefully stop XCTest agent

[mrousavy]

Summary

• add a /shutdown endpoint to the iOS XCTest agent and exit testAgentSession when Harness stops the session
• request graceful shutdown from the platform-ios controller before terminating xcodebuild
• bound shutdown with a timeout and fall back to SIGTERM/SIGKILL if the agent does not stop

Validation

• pnpm exec vitest run --root packages/platform-ios src/__tests__/xctest-agent-client.test.ts src/__tests__/xctest-agent.test.ts
• pre-commit lint/typecheck hook
• pre-push build hook

Context

This prevents normal Harness teardown from killing a passing xcodebuild test-without-building session with SIGTERM after the JS tests complete.

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