Improve argument decoding error handling in ExecuteToolCommand

[AmadulHaque]

Summary

Improve input validation for the boost:execute-tool command by validating the Base64-encoded arguments before attempting to decode the JSON payload.

Changes

• Validate the result of base64_decode() before calling json_decode().
• Return a clear error when the provided arguments are not valid Base64.
• Keep existing JSON validation for malformed payloads.
• Update tests to cover the new validation behavior and remove the test that expected a TypeError.