fix(cursor): release discarded OAuth poll responses

[ooiuuii]

Problem

Fixes #571.

The Cursor browser-auth poller leaves non-2xx response bodies unread. Pending 404 responses and error responses can retain pooled connection resources and delay the next poll.

Value

Keep a waiting browser login from unnecessarily blocking its own subsequent requests in a constrained connection pool.

Approach

Cancel discarded response bodies before the existing status handling. Ignore cleanup rejection so it cannot replace the established pending/error semantics. The same cleanup idiom already exists in the usage provider utilities.

Only four production lines are added. Retry count, backoff, request/login deadlines and credentials remain unchanged. The existing request timeout bounds retention; this is not a claim of permanent leakage or universal login failure.

Validation

• Real loopback HTTP fixture + single-connection Undici pool: unchanged poller blocks before request two; fixed poller reaches the synthetic success response. No real provider/account data.
• Exact final tests against unchanged production: 2 fail / 1 pass; fixed 3/3 pass, including error cleanup and cleanup-rejection behavior.
• Complete provider Node tests: 88/88 pass.
• Bun mock-body cases: 2/2 pass. The actual pool fixture uses the repository's Node runner; Bun's Undici shim lacks Pool.destroy, so a full bun test run of this file is not claimed.
• bun run check, git diff --check, one scoped read-only review — pass/no actionable findings.
• Full local suite not claimed green because this Windows host has reproduced baseline timing failures and a background-terminal cleanup hang. GitHub CI will provide fresh branch-specific full-suite results.

Impact

• User-visible: avoid unnecessary connection-pool stalls during Cursor login.
• Model/tools: none.
• Runtime/lifecycle: discarded HTTP body resources are released earlier.
• Persisted config/data: none.
• Compatibility/risk: no authentication protocol, retry policy, timeout, or successful token parsing changes.