feat: wire schema_extensions from config file and preserve union keyword during merge

The merge function was converting anyOf→oneOf which broke discriminator
auto-detection, generating untagged enums instead of internally tagged.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: lightsofapollo

src/analysis.rs

@@ -393,17 +393,26 @@ fn merge_json_objects_with_rules(
     match (main, extension) {
         // Both objects - merge properties
         (Value::Object(mut main_obj), Value::Object(ext_obj)) => {
-            // Special handling for schema objects with oneOf/anyOf variants
+            // Special handling for schema objects with oneOf/anyOf variants.
+            // Detect which keyword the MAIN spec uses so we preserve it after merging.
+            let main_union_keyword = if main_obj.contains_key("oneOf") {
+                Some("oneOf")
+            } else if main_obj.contains_key("anyOf") {
+                Some("anyOf")
+            } else {
+                None
+            };
             if let (Some(main_variants), Some(ext_variants)) = (
                 extract_schema_variants(&Value::Object(main_obj.clone())),
                 extract_schema_variants(&Value::Object(ext_obj.clone())),
             ) {
+                let union_key = main_union_keyword.unwrap_or("oneOf");
                 println!(
-                    "🔍 Merging union schemas: {} main variants, {} extension variants",
+                    "🔍 Merging union schemas ({union_key}): {} main variants, {} extension variants",
                     main_variants.len(),
                     ext_variants.len()
                 );
-                // Merge the variant arrays and use oneOf as the canonical key
+                // Merge the variant arrays, preserving the original union keyword
                 // First, collect main variants, but filter out any that will be replaced by extension
                 let mut merged_variants = Vec::new();
                 let extension_refs: Vec<String> = ext_variants
@@ -438,10 +447,10 @@ fn merge_json_objects_with_rules(
                     merged_variants.push(ext_variant);
                 }
 
-                // Remove old oneOf/anyOf keys and add the merged oneOf
+                // Remove old oneOf/anyOf keys and add merged variants under the original keyword
                 main_obj.remove("oneOf");
                 main_obj.remove("anyOf");
-                main_obj.insert("oneOf".to_string(), Value::Array(merged_variants));
+                main_obj.insert(union_key.to_string(), Value::Array(merged_variants));
 
                 // Merge other properties normally
                 for (key, ext_value) in ext_obj {

src/bin/openapi-to-rust.rs

@@ -78,9 +78,20 @@ async fn main() -> Result<(), Box<dyn std::error::Error>> {
                 serde_json::from_str(&spec_content)?
             };
 
-            // Analyze schemas
+            // Analyze schemas (with extensions if configured)
             println!("🔍 Analyzing schemas...");
-            let mut analyzer = SchemaAnalyzer::new(spec_value)?;
+            let mut analyzer = if generator_config.schema_extensions.is_empty() {
+                SchemaAnalyzer::new(spec_value)?
+            } else {
+                println!(
+                    "📎 Merging {} schema extension(s)",
+                    generator_config.schema_extensions.len()
+                );
+                SchemaAnalyzer::new_with_extensions(
+                    spec_value,
+                    &generator_config.schema_extensions,
+                )?
+            };
             let mut analysis = analyzer.analyze()?;
 
             println!("📊 Found {} schemas", analysis.schemas.len());

src/config.rs

@@ -169,6 +169,10 @@ pub struct GeneratorSection {
     pub output_dir: PathBuf,
     #[validate(length(min = 1, message = "module_name cannot be empty"))]
     pub module_name: String,
+    /// Schema extension files to merge into the main spec before codegen.
+    /// Paths are relative to the working directory (same as spec_path).
+    #[serde(default)]
+    pub schema_extensions: Vec<PathBuf>,
 }
 
 #[derive(Debug, Clone, Deserialize, Serialize, Validate)]
@@ -533,7 +537,7 @@ impl ConfigFile {
             },
             streaming_config,
             nullable_field_overrides: self.nullable_overrides,
-            schema_extensions: vec![],
+            schema_extensions: self.generator.schema_extensions,
             http_client_config,
             retry_config,
             tracing_enabled,