Add a blog post about improved error handling for oauth2 configuration without internet

blog/2025-11-05-config-entry-oauth2-error-handling.md

@@ -0,0 +1,23 @@
+---
+author: Will Moss
+authorURL: https://github.com/wmoss
+title: "Improved error handling for oauth2 configuration without internet"
+---
+
+Integrations using [Application Credentials](https://developers.home-assistant.io/docs/core/platform/application_credentials/) and [Configuration via OAuth2](https://developers.home-assistant.io/docs/config_entries_config_flow_handler/#configuration-via-oauth2) need to update their error handling to correctly handle configuration when the internet is down.
+
+Currently integrations using configuration via OAuth2 call `config_entry_oauth2_flow.async_get_config_entry_implementation` in `async_setup_entry` in their `__init__.py`. Previously when there was no network, this would raise `ValueError: Implementation not available`, which was a non-retryable error, resulting in the integration needing to be manually reconfigured after internet was restored (see Issues [153956](https://github.com/home-assistant/core/issues/153956) and [144582](https://github.com/home-assistant/core/issues/144582)). [core PR 154579](https://github.com/home-assistant/core/pull/154579) added `config_entry_oauth2_flow.ImplementationUnavailableError` and raises it in `config_entry_oauth2_flow.async_get_config_entry_implementation` when OAuth2 configuration is unavailable because of missing internet. Integrations should catch this error and raise `ConfigEntryNotReady`.
+The changed behavior with the new exception will be released in 2025.12.
+
+Here is an example of the migration,
+```diff
+-    implementation = await async_get_config_entry_implementation(hass, entry)
++    try:
++        implementation = await async_get_config_entry_implementation(hass, entry)
++    except ImplementationUnavailableError as err:
++        raise ConfigEntryNotReady(
++            "OAuth2 implementation temporarily unavailable, will retry"
++        ) from err
+```
+
+New integrations will find the correct `try` / `except` block generated by `python3 -m script.scaffold config_flow_oauth2`.