Route Foundation Models through Conduit

[christopherkarani]

Summary

• route the default Foundation Models provider path through Conduit instead of Swarm's private LanguageModelSession wrapper
• make capability reporting authoritative so runtime-disabled streaming tool support is not reintroduced by protocol inference
• add provider-selection coverage for the Conduit-backed Foundation Models path and fix the MCP test compile break needed for verification

Verification

• swift test --filter 'ConduitProviderSelectionTests|AgentDefaultInferenceProviderTests|ConduitInferenceProviderTests|ConduitToolCallStreamingBridgeTests|FoundationModelsToolCallingTests'

Notes

• This is compatible with the current published Conduit graph.
• A deeper local package-edit verification against the patched Conduit worktree exposed unrelated pre-existing MLX-gating issues on Conduit main, so that dependency-side cleanup should land separately.