Deprecate Usage in favour of RequestUsage and RunUsage to work with genai-prices

[samuelcolvin]

work to support https://github.com/pydantic/genai-prices.

Closes #2352

[hyperlint-ai]

PR Change Summary

Introduced a new Usage interface to enhance compatibility with the genai-prices library.

• Updated the Usage interface to replace 'request_tokens' and 'response_tokens' with 'input_tokens' and 'output_tokens'.
• Modified multiple documentation files to reflect the new Usage structure.
• Ensured consistency across examples and usage outputs in the documentation.

Modified Files

• docs/agents.md
• docs/direct.md
• docs/message-history.md
• docs/models/index.md
• docs/models/openai.md
• docs/multi-agent-applications.md
• docs/output.md
• docs/testing.md
• docs/tools.md

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