fix(db): avoid detached datasource in SQL Server pool creator

[xuwei-fit2cloud]

Fixes #1330.

A SQL Server connection pool created from a request-scoped CoreDatasource retained that ORM object in its connection creator. After the request session closed, a later pool recycle or reconnect could read the expired ds.type attribute and raise DetachedInstanceError. Capture the datasource type while building the engine so future connections use a plain value.

The regression test creates the pool from a session-bound datasource, commits or rolls back and closes that session, then forces a reconnect from a later request. Both paths pass. The three existing pool manager tests also pass (5 total).

The same lifecycle was exercised with a reachable SQL Server instance (major version 16) through SQLBot's exec_sql: the original code reproduced DetachedInstanceError after forced recycle, while the fix returned the read-only query result. No database writes were performed. SQL Server 2019 and the full MaxKB-to-MCP workflow were not available for end-to-end verification.

[Copilot]

Copilot review overview

🟢 Approval recommended

The lifecycle fix is covered by regression tests with no unresolved issues.

Review effort: Lite
Findings: None

What changed in this PR

Fixes SQL Server pool reconnect failures after a request-scoped datasource session closes.

Changes:

• Captures the datasource type as a plain value.
• Adds commit and rollback lifecycle regression tests.

File	Summary
backend/​tests/​test_sqlserver_pool_lifecycle.py	Tests reconnect behavior after session closure.
backend/​apps/​db/​db.py	Prevents detached ORM access during pool reconnection.

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