Set `gen_ai.tool.call.id` on execute_tool spans and in message formatting

[ericapisani]

Extract tool_call_id from pydantic-ai's ToolCallPart and set it in two places:

1. On gen_ai.execute_tool spans in sentry_sdk/integrations/pydantic_ai/spans/execute_tool.py
2. In chat message serialization (tool call messages) in sentry_sdk/integrations/pydantic_ai/spans/ai_client.py

This helps correlate tool calls with their returns in multi-tool scenarios. The attribute is recommended by OTel.

[linear-code]

PY-2240