fix(e2e): stop the cap-eviction scenario from stampeding cold DO starts

[ra-co88]

What

Fixes the E2E (cloud 13of16) shard failure that's been red on main: cloud/mcp-session-cap-eviction.test.ts failing with openSession (session-N): no mcp-session-id header.

Why it fails

The scenario opens cap + 10 sessions at concurrency 8 to force a resident-runtime cap eviction. Every open is a cold Durable Object start — sqlite open plus runtime construction inside the agents SDK's blockConcurrencyWhile. Under that burst the start-up block regularly outlives workerd's wall-clock budget for it, and the platform resets the object mid-initialize. The in-flight initialize then surfaces as the restart envelope (503, JSON-RPC -32001, "MCP session is restarting, please retry") instead of a response carrying mcp-session-id — and the test failed on the very first one instead of honoring it.

Server-side evidence from a local repro: the dev server's log showed concurrency_reset platform failures for nearly every session in the burst, each correctly answered with the retryable 503 envelope by agent-handler's classifier.

What changed

Two changes, root cause first:

• Concurrency 8 → 2 for the opening Effect.forEach. Cold DO stops overlapping into reset territory. The scenario goes from a guaranteed failure (180s timeout or first-reset throw) to a ~5s pass.
• openSession honors the restart envelope. On the documented 503 "MCP session is restarting, please retry" response it retries the same initialize after a short delay (bounded, 8 attempts) instead of treating a retryable platform blip as a setup precondition failure — the same contract a real streamable-http client follows. Kept as a backstop for the occasional reset that still slips through.

Verification

• Local repro on current main: failed 2/2 before the change, both times with the 503 restart envelope (sessions 4–7 in one run, 5 in the other).
• With the change: 4/4 consecutive green runs, ~5s each.
• Sibling shard members (mcp-priming-reconnect, session-gate) still green.
• oxlint and tsc --noEmit clean on the touched package.