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/migrations/000526_boundary_log_owner.down.sql

@@ -0,0 +1,2 @@
+ALTER TABLE boundary_logs DROP CONSTRAINT IF EXISTS boundary_logs_owner_id_fkey;
+ALTER TABLE boundary_logs DROP COLUMN IF EXISTS owner_id;

coderd/database/migrations/000526_boundary_log_owner.up.sql

@@ -0,0 +1,14 @@
+ALTER TABLE boundary_logs ADD COLUMN owner_id UUID;
+
+COMMENT ON COLUMN boundary_logs.owner_id IS 'The ID of the user who owns the workspace. NULL for logs inserted before this column existed or if the user was deleted.';
+
+-- Backfill from sessions where possible.
+UPDATE boundary_logs bl
+SET owner_id = bs.owner_id
+FROM boundary_sessions bs
+WHERE bl.session_id = bs.id
+  AND bs.owner_id IS NOT NULL;
+
+ALTER TABLE boundary_logs
+    ADD CONSTRAINT boundary_logs_owner_id_fkey
+    FOREIGN KEY (owner_id) REFERENCES users(id) ON DELETE SET NULL;