feat: add hidden command group for AI Test Authoring

[arvinddotsingh]

Description

Adds a first CLI surface for Sauce Labs AI Test Authoring, backed by
the same REST backend the VS Code and IntelliJ plugins use:

• saucectl ai testcases list|get|rename|delete — manage saved test
  cases (-o text|json, --search/--limit/--skip filters)
• saucectl ai run <testCaseID> — run a saved test case in the cloud,
  poll to completion, and exit non-zero on failure (CI-friendly);
  --async, --build, --tunnel-name, --browser/--platform overrides
• saucectl ai run-suite <testSuiteID> — run every test case of a
  suite concurrently and aggregate the results (the backend has no
  suite-level run endpoint, so the suite is fanned out client-side)

Implementation notes:

• internal/aiauthoring holds the domain types and service interfaces;
  internal/http/aiauthoring.go is the REST client (Basic auth,
  requested-by client-identity header, retryable GETs, non-retried
  mutations, enveloped-response normalization, structured
  AIAuthoringAPIError with the backend's error code/detail).
• Commands are gated on the fail-closed ai_authoring.enabled org
  entitlement, distinguishing "not in plan" from verification failures.
• Run polling uses GET testcases/{id}/runs/{runId}: the job ids in the
  run-start response are internal to the backend and not resolvable via
  the Resto/RDC job APIs; sauceJobId, url, and success appear on the
  run resource as the run progresses. VDC job URLs are constructed
  from sauceJobId because the backend omits them.
• Test cases authored with an empty (rather than absent)
  runSettings.scTunnelName fail to start with SC_TUNNEL_NOT_FOUND; runs
  neutralize such tunnels by sending an explicit scTunnelName: null.

The ai group ships hidden until the feature is ready for release.