agentHost: use vacuum into for safer sqlite migration during forking

[connor4312]

No description provided.

[Copilot]

Pull request overview

This PR updates the agent host’s session forking flow to copy the per-session SQLite database via SQLite’s VACUUM INTO, aiming to produce a consistent snapshot even while the source DB is open.

Changes:

• Add ISessionDatabase.vacuumInto(targetPath) and implement it in SessionDatabase using VACUUM INTO.
• Switch Copilot session forking to use vacuumInto instead of file-copying the DB file.
• Add a unit test validating vacuumInto produces a readable copy with the same data.

Show a summary per file

File	Description
src/vs/platform/agentHost/node/sessionDatabase.ts	Implements vacuumInto using VACUUM INTO on the underlying sqlite connection.
src/vs/platform/agentHost/common/sessionDataService.ts	Extends ISessionDatabase with the new vacuumInto API + docs.
src/vs/platform/agentHost/node/copilot/copilotAgent.ts	Updates fork logic to copy the session DB via vacuumInto.
src/vs/platform/agentHost/test/node/sessionDatabase.test.ts	Adds a test ensuring vacuumInto copies turn event IDs + metadata.
src/vs/platform/agentHost/test/common/sessionTestHelpers.ts	Updates test stub TestSessionDatabase to satisfy the new interface method.

Copilot's findings

• Files reviewed: 5/5 changed files
• Comments generated: 2