azd ai agent init: update template fetcher for unified awesome-azd templates.json

[hemarina]

Background

awesome-azd PR #841 merges the standalone agent-templates.json into the unified templates.json manifest, using a templateType: "extension.ai.agent" discriminator. Once merged (and https://aka.ms/foundry-agents is repointed at templates.json), the current azd ai agent init consumer will break because:

• The remote schema is now a single mixed array (gallery + agent entries) instead of an agent-only array.
• Field names changed: language → languages (array), framework → extensionFramework.
• Language token for C# is now dotnetCsharp (matches awesome-azd''s site convention) instead of csharp.

Tracking

• awesome-azd PR: Unify agent-templates.json into templates.json via templateType discriminator awesome-azd#841
• aka.ms redirect repoint owner: @therealjohn