fix(models): honor custom_openai metadata during environment validation

[rodboev]

Current Model.validate_environment() still lets metadata-backed custom_openai/<model> names fall through LiteLLM environment validation, so aider can report a missing OPENAI_API_KEY even when the model metadata already declares litellm_provider: custom_openai.

This keeps the fix in aider/models.py: metadata-declared custom_openai models stop forcing aider-side API-key validation, while every non-custom provider stays on the existing fast path, LiteLLM validation, AWS_PROFILE adjustment, and provider-specific checks. A prefix-only custom_openai/ name still does not bypass validation.

Closes #4955

Tests:

• python -m pytest tests/basic/test_models.py -k "custom_openai_metadata_bypasses_litellm_validation or non_custom_metadata_still_uses_litellm_validation" -v
• python -m pytest tests/basic/test_aws_credentials.py -v
• pre-commit run --all-files