CLI --load: multi-connection mode (--load-connections) to test many sessions, not one #2746

Follow-up to #2439 / #2743.

Problem

--load runs every call over the single connection the CLI already opened, so it measures concurrency within one session (documented in the CLI README). It cannot show how a server behaves with many clients at once: per-session state, session creation cost, connection limits, or contention that only shows up across sessions. For stdio this means one server process. For Streamable HTTP it means one Mcp-Session-Id.

Proposal

An opt-in --load-connections <n> (default 1, so today's behavior is unchanged). It opens n independent InspectorClient connections using the same resolved transport, headers and OAuth, spreads the workers across them, and reports one combined summary. It could optionally add a per-connection breakdown, or report connect failures separately from call errors.

Things to settle:

• stdio: n connections means n spawned server processes. That is a different test from HTTP; decide whether to allow it or reject it for stdio.
• OAuth: every connection reuses the stored token, so no extra prompts. Confirm that token refresh under concurrency doesn't race.
• How --load-concurrency relates to it: total across all connections, or per connection.
• Setup and teardown cost should stay out of the latency figures.

The engine (core/cli/handlers/load-test.ts, runLoad) already takes a call function, so the change is mostly in runToolLoad.

Priority score

Severity 3 (a real capability is missing; the workaround of running several CLI processes and merging their summaries by hand is partial) + urgency 3 (wanted this milestone) + 1 (milestoned) = 7 → Medium.