Support for Custom MCP Registry URL via Environment Variable

[karthicksndr]

Introduces an environment variable MCP_REGISTRY_CUSTOM_MCP_REGISTRY_URL to allow the registry UI to load from a custom source instead of the hardcoded default.

Motivation and Context

Currently, the MCP Registry UI is hardcoded to https://registry.modelcontextprotocol.io/. This poses a challenge for:

1. Air-gapped environments where the official URL is unreachable.
2. Enterprise deployments that require hosting a private/mirrored registry.
3. Local development where developers need to test UI changes against a local registry instance.

How Has This Been Tested?

• Local docker run with the ENV var

• Local docker run without the ENV var, loads the https://registry.modelcontextprotocol.io/ by default

Breaking Changes

None.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update

Checklist

• I have read the MCP Documentation
• My code follows the repository's style guidelines
• New and existing tests pass locally
• I have added appropriate error handling
• I have added or updated documentation as needed

Additional context

#948