c85d2f9bc added 'server_error' and 'an error occurred while processing'
to the retryable message patterns to fix#3799 - the OpenAI streaming
server_error case where runtime fallback never fired. The package
layering refactor (2748009ff) moved model-error-classifier into
packages/model-core but dropped these two patterns during the move.
Restore both patterns at the matching position. packages/model-core
tests now pass on the server_error retryable assertion, and runtime
fallback once again retries the OpenAI streaming server_error envelope.
YeonGyu-Kim