JSON Schema and Validator for Activity Protocol

[varuntayur]

1. Adds a JSON schema for the activity protocol (which is in prose format). Generated from Gemini 2.5 and Verified with Claude Sonnet 4.
2. Adds C# and JS validators as reference.
3. Comes with sample messages that represent some of the important message types.

Contributors: Guruprasad Bangalore

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[gurubhg]

1. Should we integrate the activity schema validation code within Microsoft.Agents.Core, where the activity schema is already implemented
2. We should include a sample validator code. Additionally, can we add a Validate method directly in the activity schema implementation so that consumers can easily call it when they depend on Microsoft.Agents.Core?
3. We could introduce an interface with a set of methods for validating activity messages against the schema. This would allow consumers to override the default validation logic if necessary.
4. We can start with a C# and JavaScript validator, ensuring the design is flexible and easily extendable for other languages as needed.
5. We can document about the validator within the schema itself and provide a reference to the validator implementation. Additionally, we could include a sample validator in the Agents repository for clarity.