Provider conformance: add one local-server response fixture

[OthmaneBlial]

Goal

Add one synthetic Ollama or LM Studio style response fixture to the loopback OpenAI-compatible conformance suite.

Allowed files

• tests/test_providers.py
• storyboard_studio/providers.py only for a demonstrated compatibility defect
• docs/PROVIDER_POLICY.md only if supported-state wording changes

Fixture and network boundary

Use a local fake HTTP server and synthetic JSON. Never call a remote provider, include an API key, or widen the transferred fields.

Acceptance criteria

• The fixture is deterministic and licensed for repository use.
• The test proves loopback-only endpoint enforcement.
• Files, assets, evidence, sources, and speaker notes remain excluded.
• Structured output is normalized or fails with a stable safe code.
• Timeout and fallback metadata remain explicit.
• make lint, make format-check, and make test pass.

Maintainer availability

Post the target local server and response shape here first. A maintainer will confirm that it belongs in the supported conformance boundary.

Out of scope

Remote OpenAI-compatible hosts, provider ranking, streaming, or new credential storage.