docs: Explain tool execution, validation, and retries

[angelol]

This PR adds a new section to the docs/tools.md documentation explaining the process of tool execution, validation, and retries within PydanticAI.

Justification: Currently, information about how tool execution errors (like ValidationError) and explicit retries (ModelRetry) are handled is not easily discoverable via documentation search. I personally was looking for this information in the tools section of the documentation and couldn't find it even after a few tries (yes, I'm stupid). Adding this dedicated section aims to improve findability and provide users with a clearer understanding of how tools handle errors and retries during execution.

Key points covered:

• How tool arguments provided by the LLM are validated against the function signature using Pydantic.
• The automatic generation of RetryPromptPart when a ValidationError occurs, allowing the LLM to correct parameters.
• Introduction of the ModelRetry exception for tools to explicitly request a retry from their internal logic (e.g., for transient errors or invalid inputs not caught by type validation).
• Clarification that both ValidationError and ModelRetry respect the configured retries setting.

[hyperlint-ai]

PR Change Summary

Enhanced documentation on tool execution, validation, and retries in PydanticAI.

• Added a new section explaining tool execution and validation processes.
• Clarified handling of validation errors and retries with examples.
• Introduced the concept of ModelRetry for explicit retry requests during tool execution.

Modified Files

• docs/tools.md

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