Add tool timeout support

[DEENUU1]

Add tool timeout

Summary

This PR adds tool timeout support for function tools. When a tool exceeds timeout, a RetryPromptPart is returned to the model, enabling the model to try a different approach.

Features

• Agent-level timeout: Set default timeout for all tools via
  Agent(tool_timeout=60)
• Per-tool timeout: Override default with @agent.tool(timeout=30)
• Backward compatible: Default is None (no timeout)
• Function tools only: MCP servers have their own timeout mechanisms

Usage

from pydantic_ai import Agent

# Agent-level timeout applies to all tools
agent = Agent('openai:gpt-4o', tool_timeout=60)

@agent.tool_plain
async def tool_with_default_timeout() -> str:
# Will timeout after 60 seconds (agent default)
    ...

@agent.tool_plain(timeout=10)
async def tool_with_custom_timeout() -> str:
# Will timeout after 10 seconds (overrides agent default)
    ...

Implementation

• Timeout logic in FunctionToolset.call_tool() using anyio.fail_after()
• Agent passes tool_timeout to FunctionToolset as default_timeout (same pattern as max_retries)
• Per-tool timeout takes precedence over agent-level default
• On timeout, returns RetryPromptPart with message: "Timed out after {timeout} seconds."
• 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_tool_with_timeout_completes_successfully - Verifies tool completes within timeout
• 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
• test_tool_timeout_exceeds_retry_limit - Verifies UnexpectedModelBehavior on retry limit exceeded
• test_agent_level_tool_timeout - Verifies agent-level timeout applies to all tools
• test_per_tool_timeout_overrides_agent_timeout - Verifies per-tool timeout overrides agent default
• test_agent_tool_timeout_passed_to_toolset - Verifies agent passes timeout to FunctionToolset