Reuse downstream MCP sessions across tool calls

[RhysSullivan]

Problem

Every MCP tool invocation dialed a fresh downstream connection and closed it after the call, so each call ran initialize and got a new Mcp-Session-Id. Downstream servers that key state by session (Render's select_workspace → list_logs, and similar) lose that state between calls: the first call succeeds, the next one lands in a brand-new session and fails.

Observed in production against Render MCP (select_workspace ok → list_logs/get_selected_workspace failing while list_deploys — which takes an explicit serviceId — survives), and reproduced wire-level: two calls in one sandbox execution arrived at the fixture on two distinct session ids.

Fix

Remote MCP invocations now lease connections from a per-plugin-instance pool:

• One idle connection kept per resolved identity (endpoint + transport + headers/query + credential); leases are exclusive per invoke, so the per-invocation elicitation and tools/list_changed handlers keep their current semantics.
• Idle sessions expire after 5 minutes (lazy eviction, no timers).
• A connection is only returned to the pool on clean release; connection-level failures and transport-level invocation failures (new transportFailure flag on McpInvocationError) close and drop it.
• A reused session rejected with HTTP 404 (the spec's session-expired answer, rejected before the tool executes) is redialed once transparently. Other failures never auto-retry, so side-effectful tools cannot double-execute.
• The pool closes through the plugin close hook. stdio transports and endpoint probing/discovery remain strictly per-call.

Tests

• New e2e scenario mcp-session-state.test.ts: a session-stateful MCP fixture (workspace-selection shape); asserts state set by call 1 is visible to call 2 and the fixture's request ledger saw exactly one Mcp-Session-Id. Failed before this change on selfhost and cloud; passes on both now.
• New connection-pool.test.ts: sequential reuse (one session), concurrent leases (no serialization), transparent redial after server-side session loss, drop-on-failure.
• elicitation.test.ts extended: per-invoke elicitation handlers ride a reused session correctly (accept then decline over one session).
• Adjacent guards re-run green: mcp-catalog-sync (list_changed + unknown-tool healing) and policy-tool-approval e2e.

Gates: format:check, lint, typecheck, full packages/plugins/mcp suite green.

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