Add unit tests for SimplePool relay connect concurrency/backoff behavior

[joelklabo]

Problem

packages/nostr-tools/src/pool.ts has critical relay-connect logic (parallel attempts, retry backoff, and log suppression) but no direct tests in the package. Regressions here can reintroduce noisy failures or slow request paths when one relay is unreachable.

Proposed work

• Add focused tests for SimplePool.ensureRelays behavior via mocked rust-sdk client.
• Verify one failing relay does not block successful relays from being used.
• Verify retry/backoff skip window behavior for failed relays.
• Verify duplicate concurrent connect attempts for same relay share in-flight connection.

Acceptance criteria

• packages/nostr-tools has tests covering relay failure/backoff/concurrency behavior.
• Tests fail if relay connections revert to serial blocking behavior.

[joelklabo]

Tests added and verified. All 6 tests pass covering: parallel connections, in-flight connection sharing, backoff skip window, and failure tracking.