In OAuth mode _get_valid_api_key() notices the configured endpoint is
not the ChatGPT backend, corrects base_url in memory and calls
_update_provider_endpoint() to write the correction back. For a global
provider that write called config.save_providers() -- a method that does
not exist. The AttributeError was swallowed by the surrounding
except and logged as a warning, so the correction only ever lived in
memory: providers.json kept the wrong endpoint and every restart re-ran
the same dance. codex_think sat like that, configured for
api.openai.com/v1 while every request went to chatgpt.com.

Rewrite providers.json in place instead, merging into the on-disk
document rather than serialising the in-memory config, so a concurrent
dashboard edit is not clobbered by stale state. The write goes through a
temp file and os.replace() so a crash mid-write cannot truncate the
config. Missing provider is now logged explicitly rather than silently
creating an entry, and the except logs a traceback so the next failure
of this kind is not invisible.

Bump version to 0.99.92.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>