Two bugs made codex_think advertise a context window it was not
configured with, so clients that size their context from the model
listing refused to run against it.

1. The endpoint-level model cache was keyed on type+endpoint alone.
   codex_think, openai_think and bigscreen are all
   codex:https://api.openai.com/v1 but authenticate as different
   accounts, so whichever prefetched first populated the shared entry
   and the others served its model list -- an OAuth ChatGPT provider
   inherited an API-key provider's generic OpenAI models, contexts and
   all. Same collision across the three kilo-* providers. Key the entry
   on a digest of the provider's credentials too. Also fix
   invalidate_provider_cache(), which tried to find a provider's
   endpoint entry with a substring match on a key that never contains
   the provider id.

2. Provider models were published exactly as the upstream API returned
   them, so default_context_size / default_max_request_tokens never
   reached the listing. Stamp them on, letting explicit configuration
   override the fetched value.

_configured_context_size() deliberately does not call
get_context_config_for_model(): that helper ends in
_infer_context_size_from_model(), whose generic 8192 fallback is right
for sizing a request but would overwrite a real fetched window (272000)
with a guess when published.

Bump version to 0.99.89.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>