perf(provider-acp): run MCP-over-ACP calls concurrently after the handshake

One lock covered each whole POST and response read, so a short call on a
connection waited for any long call, such as t3_thread_wait, to finish.
Keep the lock only until initialize negotiates a protocol version.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
1a42091e68Victor Vogel committed on 10/22/2026, 5:39:41 PMparent7921ee7

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