Show description for anyOf fields in JSON editor

[DaleSeo]

Summary

This fixes the issue where tool parameters with anyOf schemas didn't show any description text, which left users without guidance on acceptable values. This PR adds placeholder description text to make it consistent with the fields of other types.

Type of Change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Documentation update
• Refactoring (no functional changes)
• Test updates
• Build/CI improvements

Changes Made

Adds placeholder prop support to JsonEditor component and passes schema description as placeholder to JsonEditor.

Related Issues

Only addresses the issue of missing description for anyOf fields in #915.

Testing

• Tested in UI mode
• Tested in CLI mode
• Tested with STDIO transport
• Tested with SSE transport
• Tested with Streamable HTTP transport
• Added/updated automated tests
• Manual testing performed

Test Results and/or Instructions

Tested with the following tool with anyOf input schemas:

{
  "name": "test_anyof",
  "description": "Test tool with anyOf string|integer parameter",
  "inputSchema": {
    "type": "object",
    "properties": {
      "factor": {
        "anyOf": [
          {
            "type": "string"
          },
          {
            "minimum": 1,
            "type": "integer"
          }
        ],
        "default": 1,
        "description": "Either a string or positive integer"
      }
    }
  },
  "outputSchema": {
    "type": "object",
    "properties": {
      "result": {
        "type": "string"
      }
    },
    "required": [
      "result"
    ],
    "x-fastmcp-wrap-result": true
  },
  "_meta": {
    "_fastmcp": {
      "tags": []
    }
  }
}

Before	After

Checklist

• Code follows the style guidelines (ran npm run prettier-fix)
• Self-review completed
• Code is commented where necessary
• Documentation updated (README, comments, etc.)

Breaking Changes

None. This is a pure UI enhancement that adds missing information.

Additional Context

[DaleSeo]

Hi @olaservo, when you get a chance, could you take a look at this PR? Thanks!