feat(sdk): resilient SSE reconnection in chat transport

Replace the legacy 5-attempt retry cap on SSEStreamSubscription with
indefinite retry on a bounded jittered backoff. Adds a force-reconnect
path so the chat transport can recover from silent-dead-socket cases
on mobile (background-kill, bfcache restore) without waiting for the
next backoff slot.

SSEStreamSubscription:
  - maxRetries default Infinity (was 5), retryDelayMs 100ms (was 1s),
    new maxRetryDelayMs cap (5s), retryJitter 50%
  - retryNow(): wake an in-flight backoff
  - forceReconnect(): drop current connection AND wake backoff
  - fetchTimeoutMs (30s default): aborts stuck connect attempts that
    block forever on dead sockets
  - stallTimeoutMs (opt-in): force reconnect on silent reader
  - nonRetryableStatuses (default [404, 410]): short-circuit retry
    for stream-gone / session-closed
  - Fixed listener leak where each retry accumulated an abort listener
    on the user signal because finally only ran once the recursion
    unwound. Cleanup now runs per-attempt via cleanupAttempt() in both
    the catch (before recursion) and finally paths.

TriggerChatTransport (browser):
  - online        -> forceReconnect (existing socket may be stale)
  - pageshow.persisted -> forceReconnect (Safari bfcache restore)
  - visibilitychange -> visible only:
      * hidden >= 30s -> forceReconnect
      * hidden < 30s  -> retryNow (cheap wake)
  - stallTimeoutMs: 60s (sized over typical agent thinking pauses)

Tests: 13 vitest cases covering retry-past-legacy-cap, backoff cap,
jitter variance, retryNow short-circuit, abort-during-backoff,
forceReconnect during fetch and during read (verifies Last-Event-ID
resume on the resumed request), fetchTimeout, stallTimeout, 404/410
short-circuit, custom nonRetryableStatuses, 503 still retries.

Refs TRI-8903.

Author: ericallam