fix(FN-011): wrap validate_connection in try/except, flush before return, remove unused import, document unique constraint deferral

Fusion-Task-Id: FN-011
Fusion-Task-Lineage: 4a9aca6f-9d91-43aa-8d2a-d59657c1541a

apps/api/app/git/connection.py

@@ -3,7 +3,6 @@
 import uuid
 
 from pydantic import BaseModel
-from sqlalchemy import select
 from sqlalchemy.ext.asyncio import AsyncSession
 
 from app.git.credentials import CredentialStorage, GitCredential
@@ -50,7 +49,15 @@ class ConnectionManager:
         session.add(row)
         await session.flush()
 
-        status = self.provider.validate_connection(git_url, str(credential_id))
+        try:
+            status = self.provider.validate_connection(
+                git_url, str(credential_id)
+            )
+        except Exception:
+            row.connection_status = str(ConnectionStatus.error)
+            await session.flush()
+            raise RuntimeError("Connection validation failed")
+
         if status == ConnectionStatus.connected:
             row.connection_status = str(ConnectionStatus.connected)
         else:
@@ -58,7 +65,7 @@ class ConnectionManager:
             await session.flush()
             raise RuntimeError("Connection validation failed")
 
-        await session.refresh(row)
+        await session.flush()
         return _map_row(row)
 
     async def disconnect(

apps/api/app/models/repository_connection.py

@@ -14,6 +14,9 @@ if TYPE_CHECKING:
 
 class RepositoryConnection(Base, UUIDMixin, TimestampMixin):
     __tablename__ = "repository_connection"
+    # NOTE: A partial unique index on (project_id, repository_id, provider_kind)
+    # when repository_id IS NOT NULL is deferred for MVP. Duplicate connections
+    # are acceptable until explicit disambiguation is required.
 
     project_id: Mapped[uuid.UUID] = mapped_column(
         ForeignKey("project.id"), index=True