Add web search pause_turn resubmission example

[mindbomber]

Fixes #1339

Summary

• Add a focused web-search pause_turn example that appends the paused assistant message as a complete assistant turn.
• Preserve returned server-tool content blocks instead of splitting, filtering, or rewriting individual server_tool_use blocks.
• Add regression coverage showing MessageParam request transformation preserves ServerToolUseBlock models when resubmitting assistant content.

Verification

• python -m pytest tests/test_transform.py -q -k server_tool_use -> 2 passed
• python -m ruff check tests/test_transform.py examples/web_search_pause_turn.py -> passed
• python -m py_compile examples/web_search_pause_turn.py -> passed
• git diff --check -> passed
• AANA code-change guardrail -> pass / accept, AIx 1.0, no violations

Note: python -m pytest tests/test_transform.py -q also ran, but two pre-existing Windows-only failures appeared in test_base64_file_input because the checked-out tests/sample_file.txt has CRLF line endings locally, changing the expected base64 value. The new server-tool regression test passed independently.