Finalize A2UI template schema and data-binding syntax

[nan-yu]

Establish the formal technical contract for how templates are stored and validated to ensure interoperability between the Composer and the Agent SDK.

While #365 provides an initial A2UI_COMPONENT_CATALOG, we need a reusable specification for the Template.

The schema can be stored in the template sub-package of the Agent SDK.

Proposed tasks:

• Define the template schema in the Agent SDK
• Formalize the syntax for dynamic placeholders using {{}}
• Document the template schema to standardize how the LLM returns selected template names and populated data
• Provide a catalog of 3-5 reference templates as examples.

[hrithik18k]

Hi @nan-yu, I'd love to work on this issue! I've reviewed the existing Agent SDK structure in agent_sdks/python/, the agent_development.md guide, and the parent issue #657.
My plan:

Create a template/ sub-package under agent_sdks/python/src/a2ui/ with a schema.json defining the template format
Write a template_schema.md formalizing the {{}} placeholder syntax and resolution rules
Add a "Template-based Inference" section to agent_development.md
Provide 4–5 reference template examples (restaurant card, contact lookup, product detail, weather summary)

Happy to submit a draft PR and iterate based on feedback. Could you confirm if there's an internal style guide or existing WIP for the {{}} syntax I should align with?

[hrithik18k]

PR submitted at #1276 — addressed all Gemini code review suggestions in the follow-up commit. Ready for human review whenever convenient.