Add conditional tool enabling feature to agent as tool.

[thoo]

Summary

Adds is_enabled parameter to Agent.as_tool() method for conditionally enabling/disabling agent tools at runtime. Supports boolean values and callable functions for dynamic tool filtering
in multi-agent orchestration.

Test plan

• Added unit tests in tests/test_agent_as_tool.py
• Added example in examples/agent_patterns/agents_as_tools_conditional.py
• Updated documentation in docs/tools.md
• All tests pass

Issue number

Closes #1097

Checks

• I've added new tests (if relevant)
• I've added/updated the relevant documentation
• I've run make lint and make format
• I've made sure tests pass

[seratch]

@rm-openai As we have the is_enabled flag for other types of tools now, this looks reasonable change to me. What do you think?

[thoo]

@seratch Thanks for the review! I'm happy to address any suggestions you or @rm-openai might have. This feature is particularly useful for us since we work extensively with MCP agents and need the flexibility to dynamically enable/disable tools based on runtime conditions.