[Feat]: Add Max Task Completion Time for Each Skill on Agent Card

### Is your feature request related to a problem? Please describe.

Consider that clients are smart systems and select A2A server automatically. Maybe they're sensitive to time costing. when they have multiple servers to choose from which can meet their needs, time costing becomes an import factor to decide which one to select.

### Describe the solution you'd like

Add a field max_task_completion_time (and perhaps its unit) for each skill on Agent Card. Clients who obtain the card are able to test the skill and timecost of the skills to verify the authenticity of the card offline and make a agent-selecting decision online.

### Describe alternatives you've considered

_No response_

### Additional context

_No response_

### Code of Conduct

- [x] I agree to follow this project's Code of Conduct

Look for the Agent Card schema and skill definitions in the codebase, likely in a models or schemas directory. Understand how skills are currently defined and where max_task_completion_time and its unit should be added. Check for existing validation or testing of skill performance to see how clients might verify time costs. Ensure changes align with the protocol specification for agent interoperability.

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