feat: per-request toolkit isolation for concurrent multi-user requests

[Mustafa-Esoofally]

Summary

Add clone-based isolation so each interface request (Slack, Telegram, WhatsApp) gets fresh toolkit state while sharing heavy resources by reference.

Depends on: #7376

Problem

When multiple users message a Slack bot concurrently, they share the same agent instance. Google toolkits hold mutable state (creds, service, _label_cache) that can leak between users — User A's Gmail token could be used for User B's request.

Solution

Each request clones the agent via entity.deep_copy() before calling arun(). Per-user state is reset; shared resources (DB, GoogleAuth, MCP connections) are kept by reference.

Slack message arrives
  │
  ▼
entity.deep_copy()
  ├─ GoogleAuth: SHARED by reference (DB coordinator, like MCP tools)
  ├─ GmailTools: CLONED via clone(), resets creds/service/_label_cache
  ├─ MCP tools: SHARED by reference (server connections)
  └─ DB, model: SHARED by reference (connection pools)
  │
  ▼
cloned_agent.arun(message, user_id=...)
  └─ Fresh creds loaded from DB for this user

Init-time DB wiring

_initialize_agents() wires toolkit._db = agent.db at app startup (not runtime). This ensures the OAuth callback closure has DB access before any request arrives — no app.state.db needed.

Files changed (10 files, +382 -21)

Layer	Files
Clone infra	toolkit.py (clone + _clone_reset_attrs), function.py (clone_for)
deep_copy	agent/_utils.py, team/_utils.py (share GoogleAuth by ref)
Routers	slack/router.py, telegram/router.py, whatsapp/router.py
App wiring	os/app.py (init-time _db binding)
Cookbook	slack/gmail_agent.py
Tests	test_toolkit_isolation.py

Type of change

• New feature

Checklist

• Code complies with style guidelines
• Ran format/validation scripts
• Self-review completed
• Cookbook example included
• Tests added (test_toolkit_isolation.py)
• E2E tested: Slack → OAuth URL → Google consent → DB token → retry → emails displayed

Duplicate and AI-Generated PR Check

• No existing PR addresses this
• This PR was developed with AI assistance (Claude Code)

Additional Notes

• E2E verified: Full Slack OAuth flow tested with ngrok + PgVector + Gmail API
• Pattern: GoogleAuth follows the same shared-by-reference lifecycle as MCP tools
• All interfaces covered: Slack, Telegram, WhatsApp routers all call deep_copy()