Expand model provider ecosystem

[Xunzhuo]

Goal

Expand the model provider ecosystem beyond the current provider set.

Scope

• Provider registry extensibility
• Additional hosted and local model providers
• Authentication/configuration conventions
• Capability metadata such as tool use, streaming, embeddings, context length, and cost hints
• Provider conformance tests

Acceptance Criteria

• A provider expansion plan identifies target providers and required interface changes.
• Provider capability metadata is standardized enough for routing and UI surfaces.
• At least one new provider can be added through the standard provider path.
• Conformance tests make provider behavior comparable.
• Documentation explains provider setup and troubleshooting.