fix(email-schedule-rescheduler): instance-based SDK client + live-verified docs (DEV-1141)

[sonamg-droid]

Follow-up to #210 (DEV-1141). Live-review of the merged sample found that live mode was broken and one CI gate was stale. This PR fixes the code and documents the verified platform limitation.

Code fixes (live mode crashed on the first call)

app.py used the SDK 2.x static pattern, which does not exist in telnyx==4.181.0 (AttributeError: module 'telnyx' has no attribute 'EmailMessage'); demo mode (DEMO_MODE=true) masked all of it:

• telnyx.api_key = ... + telnyx.EmailMessage.create/retrieve/delete_schedule → telnyx.Telnyx(api_key=...) instance client (client.email_messages.*)
• except telnyx.error.TelnyxError → from telnyx import APIError (telnyx.error was removed in v4)
• message.id / message.scheduled_at → response.data.id / response.data.scheduled_at (responses are wrapped)
• scheduled_at exact-string compare → instant-based _same_instant() (live SDK returns a parsed datetime; the API normalizes timestamps to microsecond-Z format — the old check could never pass)
• Cleanup (DELETE schedule) now runs in finally so a failed step can never leave a scheduled email behind — previously the message would have actually sent
• Step-3 assertion now checks 422 + non-empty errors array referencing the scheduled_at field (live API does not echo the timestamp value; verified on POST which shares the validation)

Live verification (2026-09-24, account shared-domain sender)

Acceptance criterion	Result	Evidence
POST /v2/email_messages + future scheduled_at → 202 + message ID	✅ passes live	202, status: "scheduled", message id returned
PATCH /v2/email_messages/{id}/schedule → 200 + updated scheduled_at	❌ blocked (platform)	live API returns 404 code 10005 ("URL could not be found") on the documented path
Past scheduled_at → 422, no immediate send	❌ blocked same reason	can't reach a 422 on PATCH; the identical rejection does work on POST (422 "scheduled_at must be in the future", pointer /data/attributes/scheduled_at)
GET /v2/email_messages/{id} reflects new scheduled_at	⚠️ untestable until PATCH ships	GET works; SDK .data.scheduled_at verified

Route-level proof that PATCH is not deployed: DELETE /v2/email_messages/{id}/schedule on an invalid UUID returns 10001 "The requested email was not found" (route exists), while PATCH on the same path returns 10005 "The requested resource or URL could not be found" (route missing) — for both a valid scheduled message and an invalid UUID.

The reschedule route is documented in the Send Email guide and OpenAPI spec (RescheduleEmailMessage) but is not served in production yet. README.md/API.md/GUIDE.md now carry a Known limitation note; app.py prints a clear BLOCKED: message, cancels the message in cleanup, and exits instead of failing with a misleading error.

Repo-gate fixes

• Regenerated catalog.json (out of sync after PR Add email-schedule-rescheduler (DEV-1141) #210 — gen_llms_txt.py --check failed on main)
• README: fixed dead Related Examples links (email-sender / email-webhook-handler don't exist) → ai-email-agent-python / email-inbox-demo; corrected Troubleshooting rows; DEMO_MODE marked optional; shared-domain from-address restriction documented
• API.md: response shapes corrected to live/spec behavior (status: "scheduled", error code 10015, 409 row, no fabricated updated_at field)
• .env.example: removed stray markdown fences
• GUIDE.md: stale 2.x SDK snippets replaced with instance-based usage
• smoke_test.py: 20/20 tests incl. new _same_instant coverage

All gates pass locally: verify.py, rewrite_repo_links.py --check, gen_llms_txt.py --check; python -m py_compile app.py clean.