fix(agent): support tool calls in fromChatMessages and fix input typing

[warun7]

Fixes #11, Fixes #41

Summary of Changes

1. Tool Calls Support in fromChatMessages (TypeScript fromChatMessages does not properly interpret function_calls #11):

   • When an assistant message contains tool calls (toolCalls or tool_calls), properly emit corresponding function_call items.
   • If the assistant message also has non-empty text content, preserve it before the function_call items; if content is empty/null, omit the blank assistant message so only the tool calls are emitted.
   • Handle both camelCase (toolCalls, toolCallId) and snake_case (tool_calls, tool_call_id) inputs.
   • Safely stringify object arguments if provided.

2. callModel Input Type Compatibility (fromChatMessages does not produce correct object for input in callModel #41):

   • Widened BaseCallModelInput['input'] to FieldOrAsyncFunction<models.InputsUnion | Item[]>.
   • Allows passing fromChatMessages(...) and fromClaudeMessages(...) directly to callModel without TypeScript errors.

3. Tests & Changeset:

   • Added unit tests covering assistant tool calls conversion, snake_case aliases, and callModel typing.
   • Added changeset for @openrouter/agent (patch).