Auto-restart session on expiry, extend test coverage to 93%, add CI workflow

[shilpi1998]

Summary

• Auto-restart on session expiry: When the SDK emits SESSION_EXPIRED, the widget silently resets to idle instead of showing an error. Clears the stored session, resets hasUserSentRef to drop the new welcome greeting, and sets status back to idle. The user's next message transparently triggers a fresh handshake via ensureConnected.
• Clear typing/streaming state on expiry: If session expires mid-response (while agent is typing or streaming), agentTyping and streamingText are now cleared by dispatching SET_ANSWER with empty content before resetting to idle — prevents the composer from staying disabled.
• Unit test coverage extended from 77% to 93%: Added 132 tests across 9 test files covering SDK modules (client, event-emitter, logger, SSE, errors), provider logic (product-context), and token parsing.
• CI workflow: Added GitHub Actions workflow (.github/workflows/ci.yml) that runs unit tests with coverage on PRs and pushes to main (Node 20.x).
• Coverage thresholds: 90% statements/functions/lines, 80% branches — enforced by vitest in CI.

Test plan

• Let a session expire (or simulate by waiting for token TTL) — widget should reset to idle, no error shown
• Send a message after expiry — fresh session starts transparently
• If session expires mid-agent-response, typing indicator and streaming text are cleared, send button re-enables
• Welcome greeting from the new session is dropped (hasUserSentRef reset)
• npm test — all 132 tests pass with coverage above thresholds
• CI check passes on the PR