fix(e2e): honor the 503 restart envelope in the cap-eviction helper

[ra-co88]

What

Adds a retry backstop to openSession in cloud/mcp-session-cap-eviction.test.ts: when the server answers initialize with the documented restart envelope (503, JSON-RPC -32001, "MCP session is restarting, please retry" — the contract agent-handler emits for a platform DO reset), the helper retries the same request after a short delay instead of failing the scenario on a retryable platform blip.

Context

The original root-cause half of this PR (opening concurrency) was superseded by #1907, which landed sequential admission and has the shard green on main. What remains here is only the retry: the exact behavior a real streamable-http client follows for that response, kept so the test's failure mode stays aligned with client behavior if the envelope ever recurs.

Verification

• cloud/mcp-session-cap-eviction passes locally on this branch (rebased onto current main).
• oxlint and tsc --noEmit clean on the touched package.

[devin-ai-integration]

Verdict: needs a decision — superseded on main, close or trim to the retry backstop.

Open 3 days (since 2026-08-30). Conflicts with main in e2e/cloud/mcp-session-cap-eviction.test.ts: #1907 ("Stabilize cloud session cap E2E", merged 2026-08-31) already dropped the opening concurrency from 8 → 1, and the E2E (cloud 13of16) shard has been green on every main run since (b5271a6f0, 6d1f670ce, 9c67fd54e). So the root-cause half of this PR (8 → 2) is already landed in a stricter form and is the source of the conflict.

What's left is the openSession retry on the 503 "MCP session is restarting, please retry" envelope. The mechanism is correct — the string and status match packages/hosts/cloudflare/src/mcp/durable-object-errors.ts:235, and 503 = retry-same-id is the documented contract in apps/cloud/src/mcp/agent-handler.ts — but with sequential admission there's no evidence the envelope still occurs in this scenario, so it's a defensive backstop rather than a fix.

Options:

1. Close as superseded by Support raw OAuth Basic credentials #1907.
2. Author rebases, drops the concurrency hunk, keeps only the openSession retry loop (and the fixed — → - comment edit should be reverted to keep the diff minimal).

Not run: lint/typecheck/e2e — the branch doesn't apply to current main and the change is e2e-only test code with no runtime effect, so nothing to verify until it's rebased. Nothing pushed.

[ra-co88]

Took option 2 — thanks for catching that #1907 superseded the concurrency half.

Rebased onto current main, dropped the concurrency hunk (and the comment rewrite), and kept only the openSession retry loop on the documented 503 restart envelope. The diff is now a single file, retry-only: on 503 + "MCP session is restarting, please retry" it retries the same initialize after a short delay (bounded, 8 attempts), otherwise unchanged. Verified locally: cloud/mcp-session-cap-eviction passes on the rebased branch, lint and tsc --noEmit clean.

Understood on the evidence point — with sequential admission the envelope is a defensive backstop, not a demonstrated fix. Keeping it because the retry mirrors what a real streamable-http client does with that exact response, so the test's failure mode stays aligned with client behavior if the envelope ever recurs.