Added example to illustrate triage agent with delegation

[sijanonly]

This PR adds a complete example demonstrating agent delegation in a medical triage workflow.

Key highlights:

• Introduces triage_agent as a master orchestrator.
• Demonstrates delegation to specialist agents (gp_agent, cardiology_agent, neurology_agent) via the consult_specialist tool.
• Demonstrates escalation to senior doctor using consult_senior_doctor for critical or ambiguous cases.
• Structured outputs: MedicalReport, TreatmentPlan, and TriageFinalOutput.
• Includes a cohesive MedicalTriageSystem coordinator that handles multiple patient inputs and maintains medical history.
• Provides a type-safe, async-ready example compatible with Pydantic AI’s structured output and tool system.
• Markdown file medical-triage-agent.md is added with usage instructions and explanations

[sijanonly]

@Kludex , Could you please have a look into this PR when you have time ?