Fix CDATA serialization containing embedded end markers

[bconnnnn]

CDATA text containing ]]> currently produces malformed XML. Split each embedded terminator across adjacent CDATA sections, preserving the complete text for both document serialization and direct XMLPrinter::PushText calls.

For example, a]]>b now serializes as <r><![CDATA[a]]]]><![CDATA[>b]]></r>.

Fixes #1097.

Validation:

• C++11 build with -Wall -Wextra -Werror -pedantic: 576 tests passed, 0 failed.
• The same regression suite against the original implementation: 569 passed, 6 failed.
• Tests cover both memory and FILE-backed printers, including repeated and suffix-only CDATA terminators.
• An independent Python XML parser preserved all text across 1,008 directed and randomized cases, including the FILE-backed printer path.
• AddressSanitizer and UndefinedBehaviorSanitizer: 576 tests passed. Leak detection was disabled because LeakSanitizer cannot inspect processes in this environment.

AI assistance: Codex helped implement and validate the change. ByteAsk web reviewed the algorithm and helped design the FILE-backed regression tests; I adapted those suggestions and compiled and ran the tests locally.