✨ feat: Add Support for customUserVar Replacement in 'args' Field

[dustinhealy]

Summary

This pull request adds support for custom user variables (e.g., {{MY_API_KEY}}) substitution in MCP server command arguments during reinitialization. Previously, The processMCPEnv function was only processing env, headers, and url fields but did not handle the args field used by MCP servers such as those generated by Smithery. During MCP server reinitialization through the UI, custom user variables in command arguments remained as unsubstituted placeholders (e.g., '{{MY_API_KEY}}' instead of the actual API key value), causing HTTP 401 authentication errors. Related to discussion #8731

Example Use Case:

mcpServers:
  context7-mcp:
    command: npx
    args:
      - '-y'
      - '@smithery/cli@latest'
      - run
      - '@author/servername-mcp'
      - '--key'
      - "{{MY_API_KEY}}" 
      - '--profile'
      - <redacted>
    customUserVars:
      MY_API_KEY:
        title: "API Key"
        description: "Enter your personal API key from <a href='https://smithery.ai/' target='_blank'>your account settings</a>"
    startup: false

Backend Changes:

• packages/api/src/utils/env.ts:

  • Args Field Processing: Added processing for the args field in processMCPEnv function to handle custom user variable substitution in MCP server command arguments by iterating through the command arguments and replacing placeholders like {{MY_API_KEY}} with user-provided values.

• packages/api/src/mcp/mcp.spec.ts:

  • Test Coverage: Added test case "should process customUserVars in args field" to verify that custom user variables are now substituted properly in the args field

Change Type

• New feature (non-breaking change which adds functionality)

Testing

• Added unit test covering custom user variables substitution for the args field
• Verified all existing MCP environment variable tests continue to pass
• Manual testing of MCP server reinitialization with custom user variables

Checklist

• My code adheres to this project's style guidelines
• I have performed a self-review of my own code
• I have commented in any complex areas of my code
• My changes do not introduce new warnings
• A pull request for updating the documentation has been submitted.