fix(copilot): add tool call circuit breakers and intermediate persistence (#12604)

## Why

CoPilot session `d2f7cba3` took **82 minutes** and cost **$20.66** for a
single user message. Root causes:
1. Redis session meta key expired after 1h, making the session invisible
to the resume endpoint — causing empty page on reload
2. Redis stream key also expired during sub-agent gaps (task_progress
events produced no chunks)
3. No intermediate persistence — session messages only saved to DB after
the entire turn completes
4. Sub-agents retried similar WebSearch queries (addressed via prompt
guidance)

## What

### Redis TTL fixes (root cause of empty session on reload)
- `publish_chunk()` now periodically refreshes **both** the session meta
key AND stream key TTL (every 60s).
- `task_progress` SDK events now emit `StreamHeartbeat` chunks, ensuring
`publish_chunk` is called even during long sub-agent gaps where no real
chunks are produced.
- Without this fix, turns exceeding the 1h `stream_ttl` lose their
"running" status and stream data, making `get_active_session()` return
False.

### Intermediate DB persistence
- Session messages flushed to DB every **30 seconds** or **10 new
messages** during the stream loop.
- Uses `asyncio.shield(upsert_chat_session())` matching the existing
`finally` block pattern.

### Orphaned message cleanup on rollback
- On stream attempt rollback, orphaned messages persisted by
intermediate flushes are now cleaned up from the DB via
`delete_messages_from_sequence`.
- Prevents stale messages from resurfacing on page reload after a failed
retry.

### Prompt guidance
- Added web search best practices to code supplement (search efficiency,
sub-agent scope separation).

### Approach: root cause fixes, not capability limits
- **No tool call caps** — artificial limits on WebSearch or total tool
calls would reduce autopilot capability without addressing why searches
were redundant.
- **Task tool remains enabled** — sub-agent delegation via Task is a
core capability. The existing `max_subtasks` concurrency guard is
sufficient.
- The real fixes (TTL refresh, persistence, prompt guidance) address the
underlying bugs and behavioral issues.

## How

### Files changed
- `stream_registry.py` — Redis meta + stream key TTL refresh in
`publish_chunk()`, module-level keepalive tracker
- `response_adapter.py` — `task_progress` SystemMessage →
StreamHeartbeat emission
- `service.py` — Intermediate DB persistence in `_run_stream_attempt`
stream loop, orphan cleanup on rollback
- `db.py` — `delete_messages_from_sequence` for rollback cleanup
- `prompting.py` — Web search best practices

### GCP log evidence
```
# Meta key expired during 82-min turn:
09:49 — GET_SESSION: active_session=False, msg_count=1  ← meta gone
10:18 — Session persisted in finally with 189 messages   ← turn completed

# T13 (1h45min) same bug reproduced live:
16:20 — task_progress events still arriving, but active_session=False

# Actual cost:
Turn usage: cache_read=347916, cache_create=212472, output=12375, cost_usd=20.66
```

### Test plan
- [x] task_progress emits StreamHeartbeat
- [x] Task background blocked, foreground allowed, slot release on
completion/failure
- [x] CI green (lint, type-check, tests, e2e, CodeQL)

---------

Co-authored-by: Zamil Majdy <[email protected]>