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/xmlerror.pxi

@@ -756,12 +756,8 @@ cdef void _receiveGenericError(void* c_log_handler, int c_domain,
             element_size = cstring_h.strlen(c_element)
             c_message = <char*>stdlib.malloc(
                 (text_size + element_size + 1) * sizeof(char))
-            if c_message is not NULL:
-                stdio.sprintf(c_message, msg, c_element)
-                c_error.message = c_message
-            else:
-                # Out of memory, report at least the original message.
-                c_error.message = msg
+            stdio.sprintf(c_message, msg, c_element)
+            c_error.message = c_message
         else:
             c_error.message = ''
     elif c_element is NULL:
@@ -987,12 +983,6 @@ ERR_VERSION_MISMATCH=109
 ERR_NAME_TOO_LONG=110
 ERR_USER_STOP=111
 ERR_COMMENT_ABRUPTLY_ENDED=112
-WAR_ENCODING_MISMATCH=113
-ERR_RESOURCE_LIMIT=114
-ERR_ARGUMENT=115
-ERR_SYSTEM=116
-ERR_REDECL_PREDEF_ENTITY=117
-ERR_INT_SUBSET_NOT_FINISHED=118
 NS_ERR_XML_NAMESPACE=200
 NS_ERR_UNDEFINED_NAMESPACE=201
 NS_ERR_QNAME=202
@@ -1043,7 +1033,6 @@ DTD_XMLID_TYPE=540
 DTD_DUP_TOKEN=541
 HTML_STRUCURE_ERROR=800
 HTML_UNKNOWN_TAG=801
-HTML_INCORRECTLY_OPENED_COMMENT=802
 RNGP_ANYNAME_ATTR_ANCESTOR=1000
 RNGP_ATTR_CONFLICT=1001
 RNGP_ATTRIBUTE_CHILDREN=1002
@@ -1255,7 +1244,6 @@ IO_ENETUNREACH=1553
 IO_EADDRINUSE=1554
 IO_EALREADY=1555
 IO_EAFNOSUPPORT=1556
-IO_UNSUPPORTED_PROTOCOL=1557
 XINCLUDE_RECURSION=1600
 XINCLUDE_PARSE_VALUE=1601
 XINCLUDE_ENTITY_DEF_MISMATCH=1602