Volvo integration stuck in setup_retry ("Unable to update data") even after full reinstall / fresh OAuth login

### The problem The Volvo integration has been stuck in setup_retry with reason Unable to update data for about two weeks. All ~78 entities for the vehicle have been unavailable the entire time. This matches the symptoms in #163343 and #158467. I've already tried both suggested workarounds and neither fixed it: Reload the config entry (homeassistant.reload_config_entry) — failed immediately with the same "Unable to update data" error, no change. Full removal + fresh reinstall, including a brand new OAuth login to Volvo (new config entry ID, so this was not a reused/stale token) — failed immediately with the exact same error, on the very first setup attempt. Since a completely fresh login fails right away, this doesn't look like a locally expired/corrupted token — it looks like either the Volvo Connected Vehicle API is rejecting/failing these requests, or there's a bug in how the coordinator calls the API. ### What version of Home Assistant Core has the issue? 2026.8.2 ### What was the last working version of Home Assistant Core? 2026.8.2 ### What type of installation are you running? Home Assistant OS ### Integration causing the issue Volvo (homeassistant.components.volvo, built-in core integration, not HACS) ### Link to integration documentation on our website https://www.home-assistant.io/integrations/volvo/ ### Diagnostics information Downloading diagnostics for the config entry itself fails: Diagnostics fetch failed (HTTP 500): API error: 500 - 500 Internal Server Error / Server got itself in trouble. No "Reauthenticate" repair/notification appears under Settings → System → Repairs, so Home Assistant itself doesn't flag this as an invalid-credentials situation. Error log entries (component homeassistant.components.volvo.coordinator): ERROR (MainThread) [homeassistant.components.volvo.coordinator] Error fetching Volvo very slow interval coordinator data: Unable to update data ERROR (MainThread) [homeassistant.components.volvo.coordinator] Error fetching Volvo medium interval coordinator data: Unable to update data No further traceback detail is surfaced beyond "Unable to update data" (the coordinator's UpdateFailed message appears to swallow the underlying API error detail). ### Example YAML snippet ```yaml ``` ### Anything in the logs that might be useful for us? ```txt ``` ### Additional information Steps to reproduce Have a working Volvo integration entry. At some point (in my case, around 2026-08-05) it starts failing with Unable to update data and stays in setup_retry. Reload the config entry → same failure. Remove the config entry entirely and re-add it with a fresh Volvo login → same failure, immediately, on the new entry. Expected behavior Either the integration recovers automatically once Volvo's API is reachable again, or — if this is a persistent account/API-key issue — Home Assistant surfaces a proper reauthentication/repair flow instead of silently retrying forever with a generic "Unable to update data" message. Additional context Vehicle VIN redacted here; happy to provide via the auto-attached diagnostics if maintainers need it. Related: #163343 (daily authentication expiry), #158467 (login failures after switching from HACS to core integration), and this community thread: https://community.home-assistant.io/t/official-volvo-integration-suddenly-stopped-working-2025-12-2/960781 — several other users report the same error and that reinstalling did not help them either.