docs: use stable Sonnet alias in examples

[ynachiket]

Closes #1596.

Summary

• replace dated claude-sonnet-4-5-20250929 model IDs in hand-written API examples with the stable claude-sonnet-4-5 alias
• leave Bedrock examples, generated model types, tests, and recorded fixtures unchanged

Validation

• python3 -m py_compile examples/thinking_stream.py examples/text_completions_demo_sync.py examples/text_completions_demo_async.py examples/tools.py examples/web_search.py examples/tools_stream.py examples/messages_stream.py examples/messages.py examples/mcp_tool_runner.py examples/text_completions_streaming.py examples/thinking.py examples/web_search_stream.py examples/images.py examples/tools_runner.py examples/tools_runner_search_tool.py
• git diff --check
• rg -n 'model="claude-sonnet-4-5-20250929"' examples || true