fix: round-trip instruction request parts

[he-yufeng]

Fixes #5696.

Summary

• add InstructionPart to the ModelRequestPart tagged union
• add a regression test covering ModelMessagesTypeAdapter.dump_json() / validate_json() for request instruction parts

To verify

• uv run python -c "from pydantic_ai.messages import ModelRequest, InstructionPart, ModelMessagesTypeAdapter; messages=[ModelRequest(parts=[InstructionPart(content='rule', dynamic=True)])]; serialized=ModelMessagesTypeAdapter.dump_json(messages); print(serialized.decode()); print(ModelMessagesTypeAdapter.validate_json(serialized))"
• uv run pytest tests/test_messages.py::TestInstructionParts -q
• uv run pytest tests/test_messages.py -q -k "InstructionParts or model_messages_type_adapter" -p no:cacheprovider
• uv run ruff check pydantic_ai_slim\pydantic_ai\messages.py tests\test_messages.py
• uv run python -m py_compile pydantic_ai_slim\pydantic_ai\messages.py tests\test_messages.py
• git diff --check

I also ran the full tests/test_messages.py; it still has unrelated local Windows/environment failures in test_document_url_formats[csv] (mimetypes returns application/vnd.ms-excel) and test_binary_content_from_path (permission denied under C:\Users\He\AppData\Local\Temp\pytest-of-He).

The secondary BinaryImage demotion mentioned in the issue is not changed here.