Add error handling for API calls in test plan tools (#742)

adding better error handling and try-catch blocks for test plans. To
match other tools like `work-items.ts`.

## GitHub issue number
#703 

## **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?**

Updated tests
Ran thru tools manually

Signed-off-by: polatengin <118744+polatengin@users.noreply.github.com>

Author: danhellem