feat: wire up Template Builder session telemetry endpoint - #27124

…endpoint

Wire up the frontend to call POST /api/v2/templatebuilder/sessions:

- wizard_entry: fired once when the builder page becomes ready
- compose_completion: fired on create success or failure, includes
  base_template_id, module_ids, duration_seconds, and success flag

Duration is tracked via Date.now() stored in the wizard state at
initialization, then computed as elapsed seconds on create submission.

Add a client-supplied session_id (UUID) that correlates wizard_entry
and compose_completion events for the same wizard visit. This enables
precise funnel analysis and abandonment detection in BigQuery without
relying on timestamp proximity heuristics.

The session ID is generated once per page mount via crypto.randomUUID()
and threaded through the wizard state. The backend falls back to
uuid.New() if the client omits it.

…requests

Reject requests with a missing session_id as 400 Bad Request via the
validate:"required" struct tag instead of silently fabricating a UUID
server-side. A missing session ID is a client bug.

Regenerate docs/reference/api/templatebuilder.md so the code sample fence
matches generator output (sh, not shell), fixing the gen and offlinedocs
unstaged-changes checks. Drop the unused export on WizardInit to satisfy
the frontend knip lint check.

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

Drop the reportSession alias in favor of calling sessionMutation.mutate
directly, and extract the duplicated compose_completion payload into a
reportCompletion helper.

…elpers

Extract reportEntry and reportCompletion helpers to dedupe the session
telemetry payloads, replacing the reportSession alias and the repeated
compose_completion mutate calls.

…lpers together

Move reportEntry and reportCompletion to component scope as useCallback
hooks so both telemetry payloads are defined side by side.
reportCompletion takes state and duration as parameters since state is
only available inside handleCreate.