fix: remove default value for format in work item field configuration (#1170)

This pull request makes a minor adjustment to the schema definition for
work item fields in `src/tools/work-items.ts`. The default value for the
`format` property has been removed, so it is now truly optional and will
not default to "Markdown" if not specified.

## GitHub issue number
#1166

## **Associated Risks**

None

## ✅ **PR Checklist**

- [x] **I have read the [contribution
guidelines](https://github.com/microsoft/azure-devops-mcp/blob/main/CONTRIBUTING.md)**
- [x] **I have read the [code of conduct
guidelines](https://github.com/microsoft/azure-devops-mcp/blob/main/CODE_OF_CONDUCT.md)**
- [x] Title of the pull request is clear and informative.
- [x] 👌 Code hygiene
- [x] 🔭 Telemetry added, updated, or N/A
- [x] 📄 Documentation added, updated, or N/A
- [x] 🛡️ Automated tests added, or N/A

## 🧪 **How did you test it?**

manual testing

Author: danhellem

src/tools/work-items.ts

@@ -877,7 +877,7 @@ function configureWorkItemTools(server: McpServer, tokenProvider: () => Promise<
           z.object({
             name: z.string().describe("The name of the field, e.g., 'System.Title'."),
             value: z.string().describe("The value of the field."),
-            format: z.enum(["Html", "Markdown"]).optional().default("Markdown").describe("the format of the field value, e.g., 'Html', 'Markdown'. Optional, defaults to 'Markdown'."),
+            format: z.enum(["Html", "Markdown"]).optional().describe("the format of the field value, e.g., 'Html', 'Markdown'. Optional, defaults to 'Markdown'."),
           })
         )
         .describe("A record of field names and values to set on the new work item. Each fild is the field name and each value is the corresponding value to set for that field."),
@@ -1030,7 +1030,6 @@ function configureWorkItemTools(server: McpServer, tokenProvider: () => Promise<
             format: z
               .enum(["Html", "Markdown"])
               .optional()
-              .default("Markdown")
               .describe("The format of the field value. Only to be used for large text fields. e.g., 'Html', 'Markdown'. Optional, defaults to 'Markdown'."),
           })
         )