## Why
Sequential dispatch lets a long-running request block unrelated health checks
and cleanup on the same connection.
## What changed
- Add `--concurrent-requests <COUNT>` for local and remote exec-server
connections, while retaining sequential dispatch when the option is omitted
or set to `1`.
- Preserve handshake ordering before enabling concurrent dispatch.
- Reserve separate capacity for status, signal, terminate, and close requests so
they remain responsive when ordinary request capacity is saturated.
- Drain queued client responses during disconnect and cancel outstanding
request tasks during connection shutdown.
## Testing
- Cover CLI parsing and concurrency-limit validation.
- Verify default sequential behavior, pipelined handshake ordering, concurrent
request progress, control-request responsiveness, and disconnect handling.
GitOrigin-RevId: 48e4b092e318204ed635543f01f9ee0e7df095fc
eeae88d8a6