feat(copilot): add session-level dry_run flag to autopilot sessions (#12582)

## Summary
- Adds a session-level `dry_run` flag that forces ALL tool calls
(`run_block`, `run_agent`) in a copilot/autopilot session to use dry-run
simulation mode
- Stores the flag in a typed `ChatSessionMetadata` JSON model on the
`ChatSession` DB row, accessed via `session.dry_run` property
- Adds `dry_run` to the AutoPilot block Input schema so graph builders
can create dry-run autopilot nodes
- Refactors multiple copilot tools from `**kwargs` to explicit
parameters for type safety

## Changes
- **Prisma schema**: Added `metadata` JSON column to `ChatSession` model
with migration
- **Python models**: Added `ChatSessionMetadata` model with `dry_run`
field, added `metadata` field to `ChatSessionInfo` and `ChatSession`,
updated `from_db()`, `new()`, and `create_chat_session()`
- **Session propagation**: `set_execution_context(user_id, session)`
called from `baseline/service.py` so tool handlers can read
session-level flags via `session.dry_run`
- **Tool enforcement**: `run_block` and `run_agent` check
`session.dry_run` and force `dry_run=True` when set; `run_agent` blocks
scheduling in dry-run sessions
- **AutoPilot block**: Added `dry_run` input field, passes it when
creating sessions
- **Chat API**: Added `CreateSessionRequest` model with `dry_run` field
to `POST /sessions` endpoint; added `metadata` to session responses
- **Frontend**: Updated `useChatSession.ts` to pass body to the create
session mutation
- **Tool refactoring**: Multiple copilot tools refactored from
`**kwargs` to explicit named parameters (agent_browser, manage_folders,
workspace_files, connect_integration, agent_output, bash_exec, etc.) for
better type safety

## Test plan
- [x] Unit tests for `ChatSession.new()` with dry_run parameter
- [x] Unit tests for `RunBlockTool` session dry_run override
- [x] Unit tests for `RunAgentTool` session dry_run override
- [x] Unit tests for session dry_run blocks scheduling
- [x] Existing dry_run tests still pass (12/12)
- [x] Existing permissions tests still pass
- [x] All pre-commit hooks pass (ruff, isort, pyright, tsc)
- [ ] Manual: Create autopilot session with `dry_run=True`, verify
run_block/run_agent calls use simulation

---------

Co-authored-by: Claude Opus 4.6 (1M context) <[email protected]>