Refactor theme validation to use catalog schema reference

[jacobsimionato]

This PR refactors the A2UI v0.9 and v0.10 protocol schemas to strictly reference theme properties from the component catalog, aligning theme validation with the existing pattern for components and functions.

Changes:

• Schema Refactoring:

  • Removed the top-level theme object from standard_catalog.json (v0.9 & v0.10).
  • Inlined primaryColor, iconUrl, and agentDisplayName into the $defs/theme definition within standard_catalog.json.
  • Updated server_to_client.json (v0.9 & v0.10) to reference the theme schema via catalog.json#/$defs/theme instead of using a generic object type.

• Documentation:

  • Updated a2ui_protocol.md (v0.9 & v0.10) to explicitly state that the theme schema is part of the catalog and referenced via the catalog.json alias.

• Testing:

  • Added comprehensive theme_validation.json test suites for both versions to verify that valid themes are accepted and invalid ones (e.g., incorrect types or hex codes) are rejected.

[gemini-code-assist]

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