Add owned stdio clients for local MCP servers

[quinnj]

Local MCP servers using stdin/stdout cannot currently use this package's client: a manually constructed stdio descriptor fails during initialization with transport_unsupported. Add the qualified ModelContextProtocol.prepare_stdio_client(command) constructor, including a do-block form, and reuse the existing initialize, list/call, handler, and termination APIs.

The client owns one local child and its stdin/stdout pipes. It correlates concurrent requests, routes responses independently of user callbacks, keeps stderr separate, bounds frames/queues/request waits, and closes/reaps the child on EOF or failure. Condition notifications remove the polling floor in the new implementation; write-deadline races preserve failure, while complete replies received before EOF remain available. Shutdown can retry completed cleanup failures. Arbitrary blocked callbacks require cooperative release and produce a bounded close timeout.

Protocol selection is explicit: 2025-11-25 by default, or 2026-07-28 discovery and request metadata. Automatic version probing/restart/replay, modern subscriptions, Agentif catalog/schema import, and native subprocess compilation are outside this change. There are no new dependencies or exports. HTTP behavior and the old 16-, 18-, and 19-argument client constructors remain covered; MCPClient gains one optional field, which changes its exact field layout.

Validation on macOS arm64:

• Full Julia 1.13 suite: 828 assertions, including 346 stdio assertions and all 7 existing strict static-server native controls.
• Full Julia 1.10 suite: 822 assertions, including the same 346 stdio assertions and its expected native-unavailable check.
• Strict documentation/doctests and the runnable local-child example pass. Local fixtures cover both protocol versions, concurrent/reentrant calls, exact ID types, cancellation/late replies, malformed/partial/oversized frames, blocked stdin/stderr/callbacks, EOF, termination escalation, and cleanup retry.
• The actual subprocess entrypoint works under Julia, but strict native compilation reports 345 verifier errors and produces no executable. No verifier suppression is used.
• Final warmed local echo medians are 148 µs on Julia 1.13 and 158 µs on Julia 1.10. The roughly 16× improvement is against the earlier polling prototype, not an existing-package capability. Warm HTTP allocations are unchanged; first HTTP call time increases by about 0.21 seconds in the final matched pair. Hosted CI passes all nine runtime/platform jobs and documentation at the published head, including Linux and Windows.

Co-authored by Codex