feat(ai): add persistent conversation store backends

[beatenevo]

Please provide a description of this PR:

This PR adds pluggable persistence for AI conversation data.

Related to #1502.

Changes

• Define SessionStore, MessageStore, and the combined Store interfaces.
• Migrate the existing in-memory session, turn, and message logic to MemoryStore.
• Inject one shared Store instance into the Session Manager, Agent, and Memory Tool.
• Add GORM models for sessions, turns, and messages.
• Implement GormStore for persistent session history.
• Add Memory/GORM backend configuration and database connection initialization.
• Add schema migration support.
• Add transactional writes and deletions.
• Enforce message ordering with a unique (turn_id, sequence) constraint.
• Delete session-related messages and turns transactionally without database foreign keys.
• Prevent expired-session cleanup from deleting sessions refreshed by another instance.
• Add contract, rollback, restart recovery, multi-instance sharing, JSON codec, and component regression tests.

Testing

The following checks pass:

• go test ./...
• go test -race ./store/... ./component/agent/react ./component/tools/test ./component/server/engine ./component/server/engine/sse
• go vet ./...

SQLite is used for local persistence and behavioral tests. MySQL and PostgreSQL drivers are included and compile successfully. Service-backed MySQL/PostgreSQL integration tests are outside the current PR because CI database services are not configured.

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• Docs
• Installation
• User Experience
• Dubboctl
• Console
• Core Component

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[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud