core: Strengthen test assertions based on review feedback

Replace weak `assert\!(matches\!(result, Token::*))` patterns with stronger
`assert_eq\!(result, tuple_fields[index])` assertions that compare actual
values. This provides better test coverage and more precise validation.

Also removes error handling and best practices sections from documentation
per feedback.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: fubhy

docs/subgraph-manifest.md

@@ -195,20 +195,6 @@ calls:
   nestedField: Contract[event.params.data.0.inner.fieldName].process()
 ```
 
-#### Error Handling
-
-When using invalid field names, Graph Node provides helpful error messages:
-
-```
-In declarative call 'myCall': unknown field 'invalid' in struct parameter 'asset'. 
-Available field names: [active, addr, amount]. Available numeric indices: [0, 1, 2]
-```
-
-**Best Practices:**
-- Use named field access (`asset.addr`) for better readability and maintainability
-- Named access requires the struct field names to be present in the ABI
-- Numeric indices (`asset.0`) work as a fallback and for backward compatibility
-- Both access methods can be mixed in the same expression for complex nested structures
 
 ## 1.6 Path
 A path has one field `path`, which either refers to a path of a file on the local dev machine or an [IPLD link](https://github.com/ipld/specs/).

graph/src/data_source/common.rs

@@ -1221,7 +1221,7 @@ mod tests {
             None,
         )
         .unwrap();
-        assert!(matches!(result, Token::Uint(_)));
+        assert_eq!(result, tuple_fields[0]);
 
         // Test accessing index 1 (address)
         let result = CallDecl::extract_nested_struct_field(
@@ -1232,10 +1232,7 @@ mod tests {
             None,
         )
         .unwrap();
-        assert!(matches!(result, Token::Address(_)));
-        if let Token::Address(addr) = result {
-            assert_eq!(addr, [1u8; 20].into());
-        }
+        assert_eq!(result, tuple_fields[1]);
 
         // Test accessing index 2 (uint256)
         let result = CallDecl::extract_nested_struct_field(
@@ -1246,7 +1243,7 @@ mod tests {
             None,
         )
         .unwrap();
-        assert!(matches!(result, Token::Uint(_)));
+        assert_eq!(result, tuple_fields[2]);
 
         // Test that it works in a declarative call context
         let expr: CallExpr = "ERC20[event.params.asset.1].name()".parse().unwrap();
@@ -1537,6 +1534,7 @@ mod tests {
         ]);
 
         // Test extracting nested field using numeric path [1, 0] (asset.token.addr)
+        let expected_result = Token::Address([0x42; 20].into());
         let result = CallDecl::extract_nested_struct_field(
             &outer_struct,
             &[FieldAccess::Index(1), FieldAccess::Index(0)],
@@ -1545,10 +1543,7 @@ mod tests {
             None,
         )
         .unwrap();
-        assert!(matches!(result, Token::Address(_)));
-        if let Token::Address(addr) = result {
-            assert_eq!(addr, [0x42; 20].into());
-        }
+        assert_eq!(result, expected_result);
     }
 
     #[test]