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>

coderd/templatebuilder_handler_test.go

@@ -5,6 +5,7 @@ import (
 	"net/http"
 	"testing"
 
+	"github.com/google/uuid"
 	"github.com/stretchr/testify/require"
 
 	"github.com/coder/coder/v2/coderd/coderdtest"
@@ -238,3 +239,120 @@ func TestTemplateBuilderModules(t *testing.T) {
 		require.Equal(t, http.StatusNotFound, sdkErr.StatusCode())
 	})
 }
+
+func TestTemplateBuilderSession(t *testing.T) {
+	t.Parallel()
+
+	t.Run("WizardEntry", func(t *testing.T) {
+		t.Parallel()
+		client := coderdtest.New(t, nil)
+		_ = coderdtest.CreateFirstUser(t, client)
+
+		ctx, cancel := context.WithTimeout(context.Background(), testutil.WaitLong)
+		defer cancel()
+
+		err := client.TemplateBuilderSession(ctx, codersdk.TemplateBuilderSessionRequest{
+			SessionID: uuid.New(),
+			EventType: codersdk.TemplateBuilderSessionEventWizardEntry,
+		})
+		require.NoError(t, err)
+	})
+
+	t.Run("ComposeCompletion", func(t *testing.T) {
+		t.Parallel()
+		client := coderdtest.New(t, nil)
+		_ = coderdtest.CreateFirstUser(t, client)
+
+		ctx, cancel := context.WithTimeout(context.Background(), testutil.WaitLong)
+		defer cancel()
+
+		err := client.TemplateBuilderSession(ctx, codersdk.TemplateBuilderSessionRequest{
+			SessionID:       uuid.New(),
+			EventType:       codersdk.TemplateBuilderSessionEventComposeCompletion,
+			BaseTemplateID:  "docker",
+			ModuleIDs:       []string{"code-server", "git-clone"},
+			DurationSeconds: 42.5,
+			Success:         true,
+		})
+		require.NoError(t, err)
+	})
+
+	t.Run("MissingSessionID", func(t *testing.T) {
+		t.Parallel()
+		client := coderdtest.New(t, nil)
+		_ = coderdtest.CreateFirstUser(t, client)
+
+		ctx, cancel := context.WithTimeout(context.Background(), testutil.WaitLong)
+		defer cancel()
+
+		err := client.TemplateBuilderSession(ctx, codersdk.TemplateBuilderSessionRequest{
+			EventType: codersdk.TemplateBuilderSessionEventWizardEntry,
+		})
+		require.Error(t, err)
+
+		var sdkErr *codersdk.Error
+		require.ErrorAs(t, err, &sdkErr)
+		require.Equal(t, http.StatusBadRequest, sdkErr.StatusCode())
+	})
+
+	t.Run("InvalidEventType", func(t *testing.T) {
+		t.Parallel()
+		client := coderdtest.New(t, nil)
+		_ = coderdtest.CreateFirstUser(t, client)
+
+		ctx, cancel := context.WithTimeout(context.Background(), testutil.WaitLong)
+		defer cancel()
+
+		err := client.TemplateBuilderSession(ctx, codersdk.TemplateBuilderSessionRequest{
+			EventType: "invalid_event",
+		})
+		require.Error(t, err)
+
+		var sdkErr *codersdk.Error
+		require.ErrorAs(t, err, &sdkErr)
+		require.Equal(t, http.StatusBadRequest, sdkErr.StatusCode())
+	})
+
+	t.Run("DisabledReturns404", func(t *testing.T) {
+		t.Parallel()
+		dv := coderdtest.DeploymentValues(t)
+		dv.TemplateBuilder.Disabled = true
+
+		client := coderdtest.New(t, &coderdtest.Options{
+			DeploymentValues: dv,
+		})
+		_ = coderdtest.CreateFirstUser(t, client)
+
+		ctx, cancel := context.WithTimeout(context.Background(), testutil.WaitLong)
+		defer cancel()
+
+		err := client.TemplateBuilderSession(ctx, codersdk.TemplateBuilderSessionRequest{
+			EventType: codersdk.TemplateBuilderSessionEventWizardEntry,
+		})
+		require.Error(t, err)
+
+		var sdkErr *codersdk.Error
+		require.ErrorAs(t, err, &sdkErr)
+		require.Equal(t, http.StatusNotFound, sdkErr.StatusCode())
+	})
+
+	t.Run("MemberCannotSubmit", func(t *testing.T) {
+		t.Parallel()
+		client := coderdtest.New(t, nil)
+		admin := coderdtest.CreateFirstUser(t, client)
+
+		memberClient, _ := coderdtest.CreateAnotherUser(t, client, admin.OrganizationID)
+
+		ctx, cancel := context.WithTimeout(context.Background(), testutil.WaitLong)
+		defer cancel()
+
+		err := memberClient.TemplateBuilderSession(ctx, codersdk.TemplateBuilderSessionRequest{
+			EventType: codersdk.TemplateBuilderSessionEventWizardEntry,
+		})
+		require.Error(t, err)
+
+		var sdkErr *codersdk.Error
+		require.ErrorAs(t, err, &sdkErr)
+		require.Equal(t, http.StatusNotFound, sdkErr.StatusCode())
+	})
+}