fix(tooling): resolve Gemini API schema validation error

[owenmillera]

This pull request resolves a schema validation error that occurs when using the Directus MCP server with the Gemini CLI. The create-field tool definition included a width parameter using z.enum, which was not correctly translated into a valid JSON schema, omitting the necessary type property. This caused a 400 INVALID_ARGUMENT error from the Gemini API.

The fix involves two main changes:

• In src/types/fields.ts, the z.enum for the width property in FieldMetaSchema and FieldSchema has been replaced with optionalString(), and the possible values are now described in the parameter's description.
• In src/utils/define.ts, explicit return types (ToolDefinition and PromptDefinition) have been added to the defineTool and definePrompt functions to resolve a TypeScript build error that surfaced after the first change.

After applying these changes, the project builds successfully, and the Gemini CLI operates without any API errors.