Add compiler error when single value event used with multiple data entries (#1794)

### What

Adds a compiler error during `derive_event` that informs the user single
value events can't have multiple data fields.

### Why

The compiler error that surfaced if multiple data fields existed for a
single value event was unclear.
```
error: expected one of `.`, `;`, `?`, `}`, or an operator, found keyword `self`
   --> soroban-sdk/src/tests/contract_event.rs:224:5
    |
224 |     #[contractevent(data_format = "single-value")]
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected one of `.`, `;`, `?`, `}`, or an operator
    |
```

The new error is:
```
error: data_format = "single-value" requires exactly 0 or 1 data fields, but found more
   --> soroban-sdk/src/tests/contract_event.rs:224:5
    |
224 |     #[contractevent(data_format = "single-value")]
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
```

Fixes #1793

### Known limitations

None

Author: mootz12

soroban-sdk-macros/src/derive_event.rs

@@ -163,7 +163,7 @@ fn derive_impls(args: &ContractEventArgs, input: &DeriveInput) -> Result<TokenSt
         .collect::<Vec<_>>();
 
     // If errors have occurred, return them.
-    let errors = errors.checkpoint()?;
+    let mut errors = errors.checkpoint()?;
 
     // Generated code spec.
     let export = args.export.unwrap_or(true);
@@ -266,10 +266,17 @@ fn derive_impls(args: &ContractEventArgs, input: &DeriveInput) -> Result<TokenSt
         DataFormat::SingleValue if data_params_count == 0 => quote! {
             #path::Val::VOID.to_val()
         },
-        DataFormat::SingleValue => quote! {
-            use #path::IntoVal;
-            #(self.#data_idents.into_val(env))*
-        },
+        DataFormat::SingleValue => {
+            if data_params_count > 1 {
+                errors.push(Error::custom(
+                    "data_format = \"single-value\" requires exactly 0 or 1 data fields, but found more",
+                ));
+            }
+            quote! {
+                use #path::IntoVal;
+                #(self.#data_idents.into_val(env))*
+            }
+        }
         DataFormat::Vec if data_params_count == 0 => quote! {
             use #path::IntoVal;
             #path::Vec::<#path::Val>::new(env).into_val(env)