Fix/add id to toolcall

[SaiDevaHarshaR]

What this fixes

Fixes #667 — ToolCall is missing id field required by OpenAI API spec.
When using ollama-python responses with OpenAI-compatible tools,
tool_call_id is missing causing errors.

Change

Added id: Optional[str] = None to Message.ToolCall:

class ToolCall(SubscriptableBaseModel):
    id: Optional[str] = None  # added
    function: Function

Why Optional

Making it optional preserves backward compatibility — existing code
that creates ToolCall without id continues to work.

Testing

# With id
tc = Message.ToolCall(id="call_abc123", function=...)
print(tc.id)  # "call_abc123"

# Without id (backward compatible)
tc = Message.ToolCall(function=...)
print(tc.id)  # None

Closes #667