feat: wire up Template Builder session telemetry endpoint (#27124)

`TemplateBuilderSession` telemetry types and telemetry-server ingestion
were added in earlier PRs (#25082, coder/coder-telemetry-server#41), but
no code ever produced session events. This adds the missing producer.

**Backend**: `POST /api/v2/templatebuilder/sessions` reports wizard
entry and compose completion events directly via
`api.Telemetry.Report()`, using the same inline pattern as
`NetworkEvents` and `UserTailnetConnections`. No database migration or
`createSnapshot()` changes needed. RBAC requires `policy.ActionCreate`
on `ResourceTemplate.AnyOrganization()`, matching the compose endpoint.

**Frontend**: The template builder wizard fires `wizard_entry` on page
mount and `compose_completion` on create success or failure. A
client-generated session ID (UUID) correlates the two events for the
same wizard visit, enabling precise funnel analysis and abandonment
detection in BigQuery. Duration is tracked via `Date.now()` in the
wizard state.

Closes https://linear.app/codercom/issue/DEVEX-599

<details>
<summary>Implementation plan</summary>

## Root Cause Analysis

The DEVEX-599 ticket diagnosis suggested missing DB tables, queries, and
`eg.Go` blocks. That diagnosis assumes the DB-backed periodic snapshot
path is required. It is not. Investigation shows two telemetry reporting
patterns in the codebase:

1. **DB-backed periodic snapshots** (`createSnapshot()` with `eg.Go`
blocks): Used for durable entities like workspaces, templates, users.
2. **Direct inline reporting**
(`api.Telemetry.Report(&telemetry.Snapshot{...})`): Used for ephemeral
events like `NetworkEvents`, `UserTailnetConnections`, `CLIInvocations`.

Template builder sessions are ephemeral events, so the direct inline
reporting pattern is the correct fit.

## Backend Changes

- `codersdk/templatebuilder.go`: `TemplateBuilderSessionRequest` type
with `SessionID`, `EventType` enum, `TemplateBuilderSession()` client
method
- `coderd/coderd.go`: Route registration in `/templatebuilder` group
- `coderd/templatebuilder_handler.go`: Handler with RBAC check, request
validation, session ID fallback, and inline telemetry report
- `coderd/templatebuilder_handler_test.go`: Tests for wizard entry,
compose completion, invalid event type, disabled feature, and member
RBAC rejection

## Frontend Changes

- `site/src/api/api.ts`: `recordTemplateBuilderSession` API method
- `site/src/api/queries/templateBuilder.ts`: React Query mutation
- `site/src/pages/TemplateBuilder/wizardState.ts`: `sessionId` and
`enteredAt` fields, `createWizardState()` factory for per-mount
initialization
- `site/src/pages/TemplateBuilder/TemplateBuilderPageView.tsx`:
`sessionId` prop, `useReducer` initializer form
- `site/src/pages/TemplateBuilder/TemplateBuilderPage.tsx`: Telemetry
calls for wizard entry (on mount) and compose completion (on create
success/failure)

</details>

> 🤖 Generated by Coder Agents

---------

Co-authored-by: Coder Agent <agent@coder.com>

codersdk/templatebuilder.go

@@ -150,6 +150,40 @@ type TemplateBuilderCreateTemplateResponse struct {
 	Template Template `json:"template"`
 }
 
+// TemplateBuilderSessionEventType enumerates the event types for
+// template builder session telemetry.
+type TemplateBuilderSessionEventType string
+
+const (
+	TemplateBuilderSessionEventWizardEntry       TemplateBuilderSessionEventType = "wizard_entry"
+	TemplateBuilderSessionEventComposeCompletion TemplateBuilderSessionEventType = "compose_completion"
+)
+
+// TemplateBuilderSessionRequest is the request body for
+// POST /api/v2/templatebuilder/sessions.
+type TemplateBuilderSessionRequest struct {
+	SessionID       uuid.UUID                       `json:"session_id" format:"uuid" validate:"required"`
+	EventType       TemplateBuilderSessionEventType `json:"event_type" validate:"required,oneof=wizard_entry compose_completion"`
+	BaseTemplateID  string                          `json:"base_template_id,omitempty"`
+	ModuleIDs       []string                        `json:"module_ids,omitempty"`
+	DurationSeconds float64                         `json:"duration_seconds,omitempty"`
+	Success         bool                            `json:"success,omitempty"`
+}
+
+// TemplateBuilderSession reports a template builder session event for
+// telemetry purposes.
+func (c *Client) TemplateBuilderSession(ctx context.Context, req TemplateBuilderSessionRequest) error {
+	res, err := c.Request(ctx, http.MethodPost, "/api/v2/templatebuilder/sessions", req)
+	if err != nil {
+		return err
+	}
+	defer res.Body.Close()
+	if res.StatusCode != http.StatusNoContent {
+		return ReadBodyAsError(res)
+	}
+	return nil
+}
+
 // TemplateBuilderCreateTemplate composes a template from a base and modules,
 // validates it via a provisioner import job, and creates the template.
 func (c *Client) TemplateBuilderCreateTemplate(ctx context.Context, req TemplateBuilderCreateTemplateRequest) (TemplateBuilderCreateTemplateResponse, error) {