Serialize concurrent first VSP compilation

[danielhmills]

Summary

• Adds a mutex to serialize the first compilation of a VSP page when multiple HTTP workers reach the compile path concurrently.
• Prevents race conditions where several workers try to create the same generated procedure before the registry and procedure catalog entries are visible to each other.
• A proxy request that re-enters ws_request is excluded from the mutex to avoid deadlock between the proxy worker and the target worker.

Note

As imitko noted on PR #1465, the VSP is a stored procedure and the compile/re-compile critical sections should ideally handle this concurrency. This HTTP-layer mutex is a stopgap. Guidance is welcome on whether the fix belongs in the procedure compile/re-compile critical sections instead.

Test plan

• VSP pages compile and serve correctly under concurrent first-access
• Proxy requests do not deadlock

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