Add tool timeout support

[DEENUU1]

Add tool timeout support

Summary

This PR adds timeout support for tool execution, allowing users to set both global and per-tool timeouts. When a tool exceeds its timeout, a RetryPromptPart is returned to the model instead of raising an exception, enabling the model to try a different approach.

Features

• Global timeout: Set a default timeout for all tools via Agent(tool_timeout=30)
• Per-tool timeout: Override the global timeout for specific tools via @agent.tool(timeout=60)
• Precedence: Per-tool timeout takes precedence over global timeout
• Backward compatible: Default is None (no timeout)

Usage

from pydantic_ai import Agent

# Global timeout for all tools (30 seconds)
agent = Agent('openai:gpt-4o', tool_timeout=30)

@agent.tool_plain
async def slow_tool() -> str:
    # Will timeout after 30 seconds (global)
    ...

@agent.tool_plain(timeout=60)
async def very_slow_tool() -> str:
    # Will timeout after 60 seconds (per-tool override)
    ...

Implementation Details

• Uses anyio.fail_after() for async timeout handling
• On timeout, returns RetryPromptPart with message: "Tool '{name}' timed out after {timeout} seconds. Please try a different approach."
• Timeout counts as a tool failure (increments retry counter)
• Added timeout field to Tool, ToolDefinition, and ToolsetTool classes

Tests Added

• test_tool_timeout_triggers_retry - Verifies slow tools return RetryPromptPart
• test_per_tool_timeout_overrides_global - Verifies per-tool > global precedence
• test_no_timeout_by_default - Verifies backward compatibility
• test_tool_timeout_retry_counts_as_failed - Verifies retry mechanism integration
• test_tool_timeout_message_format - Verifies error message format
• test_tool_timeout_definition - Verifies ToolDefinition has timeout field
• test_tool_timeout_default_none - Verifies default timeout is None