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.

codersdk/workspaceagents.go

@@ -170,6 +170,11 @@ type WorkspaceAgent struct {
 	DisplayApps              []DisplayApp              `json:"display_apps"`
 	LogSources               []WorkspaceAgentLogSource `json:"log_sources"`
 	Scripts                  []WorkspaceAgentScript    `json:"scripts"`
+	// 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.
+	Metadata []WorkspaceAgentMetadata `json:"metadata,omitempty"`
 
 	// StartupScriptBehavior is a legacy field that is deprecated in favor
 	// of the `coder_script` resource. It's only referenced by old clients.

codersdk/workspaces.go

@@ -560,6 +560,9 @@ type WorkspaceFilter struct {
 	SharedWithUser string `json:"shared_with_user,omitempty" typescript:"-"`
 	// SharedWithGroup is the group name, group ID, or <org name>/<group name> of the group that the workspace is shared with
 	SharedWithGroup string `json:"shared_with_group,omitempty" typescript:"-"`
+	// IncludeAgentMetadata expands each agent in the response with the
+	// named metadata keys. It does not filter the returned workspaces.
+	IncludeAgentMetadata []string `json:"include_agent_metadata,omitempty" typescript:"-"`
 	// FilterQuery supports a raw filter query string
 	FilterQuery string `json:"q,omitempty"`
 }
@@ -595,6 +598,9 @@ func (f WorkspaceFilter) asRequestOption() RequestOption {
 		if f.SharedWithGroup != "" {
 			params = append(params, fmt.Sprintf("shared_with_group:%q", f.SharedWithGroup))
 		}
+		for _, key := range f.IncludeAgentMetadata {
+			params = append(params, fmt.Sprintf("include_agent_metadata:%q", key))
+		}
 		if f.FilterQuery != "" {
 			// If custom stuff is added, just add it on here.
 			params = append(params, f.FilterQuery)