Add generic fallback tool-calling behavior

[michalharakal]

Summary

Support a generic tool-calling mode for models that appear compatible but do not yet have native family-specific support.

Scope

• Define a generic fallback prompt/rendering strategy
• Reuse existing parser behavior where possible
• Make fallback selection explicit in logs/demo output

Acceptance criteria

• Unknown compatible models can still attempt tool calling
• Native support remains preferred over fallback