feat: include agent metadata in workspace list responses (#27934)

Closes #27933. Related: #27897 (single-agent GET).

Agent metadata is only readable via a per-agent watch stream, so reading
it across N workspaces costs N+1 requests. This adds a batch read to the
list endpoint:

```text
GET /api/v2/workspaces?q=param:"pool=demo" include_agent_metadata:task_status
```

- New `include_agent_metadata` search key, repeatable and key-scoped. It
expands the response, it does not filter workspaces.
- `GetWorkspaces` aggregates the requested keys as JSON behind a `CASE`:
without opt-in the response is unchanged and the subquery never runs.
Runs only for the returned page, inside the same authorized query.
- Agents in the response gain `metadata`
(`[]codersdk.WorkspaceAgentMetadata`, `omitempty`), mapped by the
`workspace_agent_id` each element carries. The collection script is
omitted; it can be long.
- `codersdk.WorkspaceFilter` gains `IncludeAgentMetadata []string`.
- No wildcard, no schema change, no migration.

---

Authored by Coder Agents on behalf of @Emyrk.

site/src/api/typesGenerated.ts

@@ -10661,6 +10661,13 @@ export interface WorkspaceAgent {
 	readonly display_apps: readonly DisplayApp[];
 	readonly log_sources: readonly WorkspaceAgentLogSource[];
 	readonly scripts: readonly WorkspaceAgentScript[];
+	/**
+	 * Metadata is only populated on the workspaces list endpoint when the
+	 * request opts in with the include_agent_metadata search key, and it
+	 * only carries the requested keys. The description's script is always
+	 * empty here: it can be long, and list consumers want values.
+	 */
+	readonly metadata?: readonly WorkspaceAgentMetadata[];
 	/**
 	 * StartupScriptBehavior is a legacy field that is deprecated in favor
 	 * of the `coder_script` resource. It's only referenced by old clients.

site/src/pages/AgentsPage/AgentChatPage.tsx

@@ -811,6 +811,7 @@ type _UncoveredAgentFields = Omit<
 	| "display_apps"
 	| "log_sources"
 	| "scripts"
+	| "metadata"
 	| "startup_script_behavior"
 >;
 // If this errors, a new field was added to WorkspaceAgent.