Enhancement: Replace hardcoded App Service templates with MCP tool (apply #1949 pattern)

Context

@manvkaur's PR #1949 replaced ~9,000 lines of hardcoded Azure Functions templates with dynamic MCP tool calls (functions_template_get), with a 3-tier fallback (CDN manifest → GitHub source → known-good Azure-Samples repos). This dramatically reduced context size and maintenance burden.

App Service should adopt the same pattern.

Scope

The App Service templates introduced in #1609 / PR #1635 currently hard-code:

• 21 files under plugin/skills/azure-prepare/references/services/app-service/templates/
• ~1,995 lines of code (split across web-app.md, web-api.md, selection.md, composition.md, and recipe source/{python,nodejs,dotnet}.md files for auth/cosmos/redis/sql)

These could be replaced with an analogous app_service_template_get (or similar) MCP tool that fetches templates dynamically from Azure-Samples repos.

Proposed approach

1. Source of truth — identify or create AZD templates in Azure-Samples for each base template (web-app, web-api) × language (Python/Node.js/.NET) × recipe (auth/cosmos/redis/sql)
2. MCP tool — extend the Azure MCP server with app_service_template_get mirroring functions_template_get semantics (language, template, infrastructure, with functionFiles[]/projectFiles[] output arrays)
3. Fallback chain — same 3-tier pattern: CDN manifest → GitHub source → known-good repos
4. Skill refactor — delete the hardcoded source/{lang}.md files; keep the orchestration files (composition.md, selection.md) with MCP tool guidance like Functions does
5. Eval coverage — add cross-language eval summaries proving templates work end-to-end

Why this isn't blocking PR #1635

PR #1635 ships proven, manually-curated templates that work today. The MCP tool approach requires:

• New MCP server endpoint
• New AZD templates published to Azure-Samples
• Awesome-AZD submissions for discoverability

These prerequisites take time. We don't want to delay shipping App Service Develop equity for an optimization that depends on cross-team work. Ship #1635 first, then layer this enhancement on top.

Reference

• PR #1949: https://github.com/microsoft/GitHub-Copilot-for-Azure/pull/1949
• Functions composition.md (post-MCP): plugin/skills/azure-prepare/references/services/functions/templates/recipes/composition.md
• Parent equity issue: #1608
• Source PR for App Service templates: #1635