fix: refresh connector tools between turns in existing sessions

[apoorvgarg31]

Summary

Fixes #662.

Refresh connector and MCP tool selection before a new turn, retry, or durable resume. This lets an existing chat pick up a newly connected account or MCP server without restarting the application or replacing its worker.

• Re-read native connection state, tool enablement, and effective chat/persona restrictions.
• Reuse existing MCP transports while rebuilding the callable selection from current configuration/discovery.
• Preserve the engine, history, permission object, and unrelated/stateful tools.
• Replace the connection-tool group atomically at a turn boundary; do not replace tools while the current tool loop or question/approval is in flight.
• Apply the same preparation path to interactive, background and scheduled execution. A preparation exception reports an error rather than running the stale registry.

The source diff is limited to four backend files and one regression-test file. No UI, dependency, database, or deployment changes.

Reproduction and tests

Tested against upstream 5bc10d928e0b64aae74313349a3b17bd19643ae2 with Python 3.12 and Node 20, matching the repository CI versions.

• Unmodified upstream backend baseline: 1,955 passed, 1 skipped.
• Behavioral regression suite against the original implementation: 12 failed, 1 passed (two new-helper tests excluded from that before/after comparison).
• Fixed pytest tests/test_connection_refresh.py -q: 15 passed.
• Fixed pytest tests -q: 1,970 passed, 1 skipped.
• GUI TypeScript check: passed.
• npm test: 181 passed.
• npm run e2e: 221 passed.
• npm run build and git diff --check: passed.

The new tests include a continuously open real-backend WebSocket for both manual connection and the managed OAuth callback, disconnect/re-enable, tool/chat/persona restrictions, pending questions, background turns, retry/resume, unrelated tool preservation, and refresh failure. A real local stdio MCP server is discovered, approved, and executed after the original session has started.

GitHub CI: the upstream workflow is awaiting maintainer approval before it can run.

These are local macOS results, not a claim that GitHub CI or packaged Desktop acceptance has already passed. Upstream's existing Playwright suite uses mocked HTTP/WebSocket endpoints; the real-backend same-socket case is covered separately by the new Python test.

Before / after screenshots

As requested by the README contribution guidance. Settings connection screenshots and reproduction details are in #662.

Before: the account is connected, but the original chat's next turn still lacks Outlook search.

After: the next turn receives Outlook search and the earlier messages remain.

The screenshots use the actual upstream UI/backend and a clearly labelled deterministic diagnostic provider that reports the actual received tool schema. External identity validation is stubbed; identities/tokens are synthetic. They do not claim live Outlook OAuth or model acceptance. Images are hosted at an immutable commit on a separate evidence branch in the contributor fork, keeping binary assets out of this PR's source diff.

Scope / limitations

This is a next-turn refresh, not immediate mid-turn revocation or cancellation redesign. It intentionally does not interrupt an already-running operation. It does not force a remote MCP tools/list on every turn, or add a new MCP transport/config reload mechanism; existing explicit reconnect/discovery behavior remains authoritative. No live external OAuth account or packaged Desktop build was used for this acceptance.