Migrate from APIM to Easy Auth, align with Python sample structure

[Copilot]

Aligns the .NET sample with the Python reference implementation, replacing APIM-based authorization with Easy Auth and updating documentation/infrastructure accordingly.

Infrastructure Changes

• Removed: APIM-based authorization (apim/, app/apim-mcp/)
• Added: entra.bicep for Entra ID app registration with federated identity credentials
• Added: mcp.bicep for Function App deployment with App Service Authentication v2
• Updated: main.bicep to use Easy Auth flow, limited regions to eastasia/northeurope/westus2

Code Changes

• Program.cs: Removed FUNCTIONS_CUSTOMHANDLER_PORT env var, hardcoded to 8080
• host.json: Added configurationProfile: "mcp-custom-handler", explicit port config, anonymous auth level

{
  "version": "2.0",
  "configurationProfile": "mcp-custom-handler",
  "customHandler": {
    "description": {
      "defaultExecutablePath": "dotnet",
      "arguments": ["QuickstartWeatherServer.dll"] 
    },
    "http": {
      "DefaultAuthorizationLevel": "anonymous"
    },
    "port": "8080"
  }
}

Documentation Changes

• Replaced APIM authorization flow with Easy Auth (401 challenge → PRM → OAuth2 login)
• Added azd template scaffolding instructions for existing servers
• Updated prerequisites with specific version requirements (azd 1.17.2+, func 4.5.0+)
• Added troubleshooting for Entra app pre-authorization

Cleanup

• Removed mcp-handler/ directory (not needed for custom handlers)
• Removed ExistingServer.md (consolidated into README)

Original prompt

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This section details on the original issue you should resolve

<issue_title>Update sample with latest changes using Python as reference</issue_title>
<issue_description>Use the Python sample as a reference and do the following: https://github.com/Azure-Samples/mcp-sdk-functions-hosting-python

• Update the readme to match the Python sample's
• Update the content of the infra folder with what the Python sample has
• Remove the FUNCTIONS_CUSTOMHANDLER_PORT from the server code because that's not needed anymore. Update the host.json to include the original port used by the server instead.
• Remove the mcp-handler directory
• Anything I might've missed. Match the Python sample in terms of content, but keeping the language specific pieces of the current sample

Don't do anything else.</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Update sample with latest changes using Python as reference #13

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