docs: add native web search SDK examples

[christineschen]

Summary

• Document native OpenRouter web search support in the README and PyPI README.
• Show Chat Completions usage with components.OpenRouterWebSearchServerTool.
• Show Responses API usage with components.WebSearchServerToolOpenRouter.
• Add serialization tests for chat web-search tools, responses web-search tools, and the components.WebSearchPlugin plugin path.

Context

Linear: DEV-478, feature request to add/support OpenRouter web search server tool in the Python SDK.

The generated SDK already contains the native web-search component types; this PR makes the support discoverable and guards the expected request serialization.

Testing

• uv run python direct invocation of tests/test_web_search_tools.py test functions
• uv run --group dev pyright src
• uv run --group dev mypy src
• uv run --group dev pylint src --rcfile pylintrc
• uv build
• git diff --check

Note: pytest is not currently installed in the repo dependency groups, so the new pytest-style tests were smoke-run by importing and invoking the test functions directly.