feat(agent): scrape linked pages to ground answers in real content

Adds `_fetch_page_content()`, which downloads a URL with a
browser-like User-Agent and uses BeautifulSoup with the lxml
parser to strip scripts, navigation, and other non-content
markup, returning up to 3000 characters of readable text.

The response validator now also compares generated replies
against this scraped text, flagging named entities and claims
that do not appear in the retrieved page content.

Dependencies: adds beautifulsoup4==4.12.3 and pins lxml==5.3.0
for parser compatibility.

venv/lib/python3.11/site-packages/lxml/includes/libxml/xmlschemas.h

@@ -18,7 +18,6 @@
 
 #include <stdio.h>
 #include <libxml/encoding.h>
-#include <libxml/parser.h>
 #include <libxml/tree.h>
 #include <libxml/xmlerror.h>
 
@@ -157,26 +156,22 @@ XMLPUBFUN void
 					 xmlStructuredErrorFunc serror,
 					 void *ctx);
 XMLPUBFUN int
-	    xmlSchemaGetParserErrors	(xmlSchemaParserCtxtPtr ctxt,
+		xmlSchemaGetParserErrors(xmlSchemaParserCtxtPtr ctxt,
 					xmlSchemaValidityErrorFunc * err,
 					xmlSchemaValidityWarningFunc * warn,
 					void **ctx);
-XMLPUBFUN void
-	    xmlSchemaSetResourceLoader	(xmlSchemaParserCtxtPtr ctxt,
-					 xmlResourceLoader loader,
-					 void *data);
 XMLPUBFUN int
-	    xmlSchemaIsValid		(xmlSchemaValidCtxtPtr ctxt);
+		xmlSchemaIsValid	(xmlSchemaValidCtxtPtr ctxt);
 
 XMLPUBFUN xmlSchemaPtr
 	    xmlSchemaParse		(xmlSchemaParserCtxtPtr ctxt);
 XMLPUBFUN void
 	    xmlSchemaFree		(xmlSchemaPtr schema);
-#ifdef LIBXML_DEBUG_ENABLED
+#ifdef LIBXML_OUTPUT_ENABLED
 XMLPUBFUN void
 	    xmlSchemaDump		(FILE *output,
 					 xmlSchemaPtr schema);
-#endif /* LIBXML_DEBUG_ENABLED */
+#endif /* LIBXML_OUTPUT_ENABLED */
 /*
  * Interfaces for validating
  */
@@ -217,7 +212,7 @@ XMLPUBFUN int
 	    xmlSchemaValidateStream	(xmlSchemaValidCtxtPtr ctxt,
 					 xmlParserInputBufferPtr input,
 					 xmlCharEncoding enc,
-					 const xmlSAXHandler *sax,
+					 xmlSAXHandlerPtr sax,
 					 void *user_data);
 XMLPUBFUN int
 	    xmlSchemaValidateFile	(xmlSchemaValidCtxtPtr ctxt,