- Rewrites ai/__init__.py with _REGISTRY dict (10 entries) replacing the if/elif chain
- get_provider() accepts ProviderConfig (not raw dict), raises ValueError on unknown
- Resolves effective values: api_key fallback to "not-needed", model/base_url/context_chars
from PROVIDER_DEFAULTS when config fields are empty/zero
- SUPPORTS_JSON_MODE lookup sets supports_json_mode on GenericOpenAIProvider instances
- AnthropicProvider instantiated without base_url (Plan 03 will widen the ctor)
- provider_config.py: context_chars default changed to 0 (sentinel for PROVIDER_DEFAULTS
resolution); 8000 default was inconsistent with O(1) factory pattern (Rule 1 fix)
- Creates backend/ai/provider_config.py as a pure data module (no provider imports)
- ProviderConfig(BaseModel) with extra=forbid: provider_id, api_key, base_url, model, context_chars
- PROVIDER_DEFAULTS dict with all 10 providers and verbatim base_url/model/context_chars from RESEARCH.md
- SUPPORTS_JSON_MODE dict with gemini=False per JSON Compatibility Matrix (D-02)
curo1305