[Bugfix][Elastic EP] Clear scaling flag on failure to prevent permane…

[gagandhakrey]

Purpose

Issue

ScalingMiddleware returns 503 for all HTTP requests while the process-wide
_scaling_elastic_ep flag is set.

AsyncLLM._scale_elastic_ep() only cleared this flag on the success path. If
engine_core.commit_elastic_ep() raised an exception or the task was cancelled,
the flag remained permanently set, causing every endpoint—including
/scale_elastic_ep and /health—to return 503 until the process restarted.

Fix

Wrap the scaling operation in a single try/finally so
_scaling_elastic_ep is always cleared, regardless of success, exceptions, or
cancellation. Remove the redundant cleanup from
_drain_requests_for_elastic_ep(), leaving a single owner for the flag's
lifecycle.

Post-fix Impact

• Prevents the scaling flag from becoming permanently latched.
• Failed scaling operations are now retryable without restarting the server.
• No behavior change to the successful scaling path.

[claude]

Claude Code Review

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