feat: GET /api/v2/agent-firewall/sessions/{id} (#24814)

Add a GET endpoint at `/api/v2/agent-firewall/sessions/{id}` that
returns agent firewall session metadata (`id`, `workspace_id`,
`owner_id`, `confined_process`, `started_at`). The handler authorizes
against the `boundary_log` resource with `ActionRead` via dbauthz.

The endpoint is enterprise-only, gated behind the `FeatureBoundary`
entitlement.

The `GetBoundarySessionByID` SQL query JOINs through `workspace_agents`
→ `workspace_resources` → `workspace_builds` → `workspaces` to return
`workspace_id` and `workspace_owner_id` directly, avoiding a separate
query.

Also adds an `owner_id` column to the `boundary_logs` table (migration
000526) with a FK to `users(id)` and a backfill from
`boundary_sessions`. This enables user-scoped RBAC authorization for
`InsertBoundaryLogs` via `.WithOwner()`, ensuring workspace agents can
only insert logs for their own owner.

Depends on #24810

**RBAC behaviour:**

| Role    | Result |
|---------|--------|
| Owner   | read   |
| Auditor | read   |
| Member  | 404    |

> [!NOTE]
> This PR was authored by Coder Agents.

coderd/database/dbmock/dbmock.go

@@ -2533,10 +2533,10 @@ func (mr *MockStoreMockRecorder) GetBoundaryLogByID(ctx, id any) *gomock.Call {
 }
 
 // GetBoundarySessionByID mocks base method.
-func (m *MockStore) GetBoundarySessionByID(ctx context.Context, id uuid.UUID) (database.BoundarySession, error) {
+func (m *MockStore) GetBoundarySessionByID(ctx context.Context, id uuid.UUID) (database.GetBoundarySessionByIDRow, error) {
 	m.ctrl.T.Helper()
 	ret := m.ctrl.Call(m, "GetBoundarySessionByID", ctx, id)
-	ret0, _ := ret[0].(database.BoundarySession)
+	ret0, _ := ret[0].(database.GetBoundarySessionByIDRow)
 	ret1, _ := ret[1].(error)
 	return ret0, ret1
 }