Remove requirement for FWR to be `custom` for MCP custom-handler to run locally

[lilyjma]

Problem Statement

Currently, Functions worker runtime (FWR) must be set to custom for custom handlers that are self-hosted MCP servers (i.e., with "configurationProfile": "mcp-custom-handler") in order to run the server locally.

However, when deploying the server to Azure, FWR must be set to a specific language (such as dotnet-isolated, python) and the AzureWebJobsFeatureFlags app setting must also be added. Attempting to add the feature flag to local.settings.json locally does not work, and FWR must be set to custom. This introduces a discrepancy between local development and cloud deployment requirements, causing confusion and extra manual steps.

Requirements

• Local development consistency: When running locally, FWR should support a specific language value (e.g., dotnet-isolated) if configurationProfile = mcp-custom-handler or AzureWebJobsFeatureFlags=EnableMcpCustomHandlerPreview is present, instead of requiring FWR to be custom.
• Deployment consistency: Both local and cloud deployment flows honor the same requirements for configuration.
• Backward compatibility: Solutions do not break existing workflows for users not using MCP custom handlers, i.e. if configurationProfile in host.json doesn't have value mcp-custom-handler, then FWR should be custom.

User Experience

Projects with local.settings.json

{
  "IsEncrypted": false,
  "Values": {
    "FUNCTIONS_WORKER_RUNTIME": "dotnet-isolated",
    "AzureWebJobsFeatureFlags": "EnableMcpCustomHandlerPreview"
  }
}

and host.json...

{
   "version": "2.0",
    "configurationProfile": "mcp-custom-handler",
    "customHandler": {
        "description": {
            "defaultExecutablePath": "python",
            "arguments": ["weather.py"] 
        },
        "http": {
            "DefaultAuthorizationLevel": "anonymous"
        },
        "port": "8000"
    }
}

should run locally without error.

Validation & Diagnostics

• If FWR is set to custom when configurationProfile=mcp-custom-handler, show clear message about FWR needing to be dotnet-isolated|python|node

Acceptance Criteria

• Local development with configurationProfile = mcp-custom-handler or AzureWebJobsFeatureFlags=EnableMcpCustomHandlerPreview works with FWR set to a specific language
• Cloud deployment preserves the same configuration requirements.
• Clear diagnostic errors for invalid FWR configuration.

Work Items

• Remove requirement for FWR to be custom in local MCP custom-handler scenarios.

FYI @fabiocav @soninaren