Add source (sibling selector) support to JSON extraction strategies

Many sites (e.g. Hacker News) split a single item's data across sibling
elements. Field selectors only search descendants, making sibling data
unreachable. The new "source" field key navigates to a sibling element
before running the selector: {"source": "+ tr"} finds the next sibling
<tr>, then extracts from there.

- Add _resolve_source abstract method to JsonElementExtractionStrategy
- Implement in all 4 subclasses (CSS/BS4, XPath/lxml, two lxml/CSS)
- Modify _extract_field to resolve source before type dispatch
- Update CSS and XPath LLM prompts with source docs and HN example
- Default generate_schema validate=True so schemas are checked on creation
- Add schema validation with feedback loop for auto-refinement
- Add messages param to completion helpers for multi-turn refinement
- Document source field and schema validation in docs
- Add 14 unit tests covering CSS, XPath, backward compat, edge cases

docs/md_v2/api/strategies.md

@@ -120,7 +120,8 @@ schema = {
             "attribute": str, # For type="attribute"
             "pattern": str,  # For type="regex"
             "transform": str, # Optional: "lowercase", "uppercase", "strip"
-            "default": Any    # Default value if extraction fails
+            "default": Any,   # Default value if extraction fails
+            "source": str,   # Optional: navigate to sibling first, e.g. "+ tr"
         }
     ]
 }