Add timestamp to SystemPromptPart

[vricciardulli]

Addresses #1145

Adjusted all relevant tests and docs.

[hyperlint-ai]

PR Change Summary

Added an optional timestamp parameter to the SystemPromptPart in various documentation files.

• Introduced an optional timestamp parameter to SystemPromptPart in multiple examples.
• Updated test cases to include the new timestamp functionality.
• Modified documentation to reflect the changes in SystemPromptPart usage.

Modified Files

• docs/message-history.md
• docs/testing-evals.md
• docs/tools.md

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[Kludex]

Thanks @vricciardulli 🙏