fix(backend/copilot-bot): recover deleted chat sessions instead of erroring (#13197)

### Why / What / How

**Why:** Deleting a Copilot chat session on the web app permanently
bricks the corresponding Discord thread/DM. The bot caches the
`session_id` in Redis under its own key
(`copilot-bot:session:<platform>:<target_id>`). When the session is
deleted on the web, `delete_chat_session` destroys the DB row and clears
the *copilot* session cache — but it has no knowledge of the bot's
mapping key, so that key keeps pointing at the dead session. The next
message resolves the stale ID, `start_chat_turn` raises
`NotFoundError("Session not found.")`, and since nothing ever clears the
stale key, **every subsequent message fails identically**. Reported by
Abhi during bot testing.

**What:** `start_chat_turn` now treats an unresolved `session_id` as
"start fresh" instead of erroring — the conversation self-heals.

**How:** When the provided `session_id` doesn't resolve to a session
(deleted, expired, or otherwise gone), fall back to
`create_chat_session(...)` rather than raising. The handle then carries
the new session ID, and the existing `on_session_id` callback
(`bot_backend.stream_chat` → `handler._on_session_id`) writes it back
into the bot's Redis cache — so the thread recovers automatically on the
next message. This also covers session deletion by *any* cause, not just
the web app. `start_chat_turn` is bot-only (sole caller is the
`PlatformLinkingManager` RPC wrapper), and the `source_platform` value
introduced in #13175 is preserved.

### Changes 🏗️

- `platform_linking/chat.py` — `start_chat_turn` falls back to a fresh
session when a provided `session_id` no longer resolves, instead of
raising `NotFoundError`.
- `platform_linking/chat_test.py` — replaced
`test_existing_session_id_wrong_user_raises_not_found` with
`test_stale_session_id_falls_back_to_fresh_session`, which reproduces
the bug and verifies the recovery.

### Checklist 📋

#### For code changes:
- [x] I have clearly listed my changes in the PR description
- [x] I have made a test plan
- [x] I have tested my changes according to the test plan:
- [x] `test_stale_session_id_falls_back_to_fresh_session` fails against
the pre-fix code (reproduces the bricked thread) and passes after the
fix
- [x] Full `platform_linking/chat_test.py` suite green (4 passed),
`black` + `ruff` + `pyright` clean