refactor: Hold service pointer in database instance

[zhanglei1949]

Fix #757

[Copilot]

Pull request overview

This PR addresses Issue #757 by enforcing a single active NeugDBService per NeugDB instance, tracking service association inside the database and rejecting conflicting operations while serving.

Changes:

• Add NeugDB↔NeugDBService association tracking via an atomic active_service_, with register/unregister hooks and a HasActiveService() query.
• Reject NeugDBService creation when one is already associated with the same NeugDB, and reject NeugDB::Connect() / NeugDB::Close() while a service is active.
• Add unit tests covering single-service enforcement, init-failure rollback, and blocking connect/close while serving.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
tests/unittest/test_db_svc.cc	Adds unit tests for single-service enforcement and blocked DB operations while serving.
src/server/neug_db_service.cc	Unregisters the service from the DB on destruction.
src/main/neug_db.cc	Implements service registration tracking; blocks Connect()/Close() while serving; adjusts destructor close behavior.
include/neug/server/neug_db_service.h	Registers/unregisters service association during construction (with rollback on init failure) and documents the constraint.
include/neug/main/neug_db.h	Declares service association API and stores the active service pointer atomically.

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[Copilot]

Pull request overview

Copilot reviewed 10 out of 10 changed files in this pull request and generated no new comments.

[liulx20]

Why do we need atomic here，Is there a race condition?

[zhanglei1949]

Changed to raw pointer. The thread safety will be guarded by the mutex.

[Copilot]

Pull request overview

Copilot reviewed 10 out of 10 changed files in this pull request and generated no new comments.