Python: fix prompt template engine blocking HTML tags in content

[OiPunk]

Motivation and Context

Fixes #13632

When a prompt contains HTML tags such as <p>, <div>, <b>, etc., the
ChatHistory.from_rendered_prompt() method wraps the rendered prompt in a
<root> element and parses it as XML. Any child elements whose tag names do
not match the known SK template tags (message, chat_history) were silently
skipped, causing their text content to be lost. This meant a prompt like:

Translate: "<p>What is your name?</p>"

would arrive at the LLM as an empty message, because <p>What is your name?</p>
was discarded during XML traversal.

Description

In ChatHistory.from_rendered_prompt, when an XML child element has an
unrecognized tag (i.e. it is not message or chat_history), the element
is now serialized back to its original text representation and appended to
the preceding message content. The element's tail text is likewise kept in
the same message, preserving the full original prompt text.

Changed files:

• python/semantic_kernel/contents/chat_history.py -- handle unknown XML
  tags as literal text in from_rendered_prompt()
• python/tests/unit/contents/test_chat_history.py -- added four regression
  tests covering HTML <p>, multiple HTML tags, HTML surrounded by text,
  and SK tags mixed with HTML

Contribution Checklist

• The code builds clean without any errors or warnings
• The PR follows the SK Contribution Guidelines
• The existing tests pass and new tests have been added
• I didn't break any existing functionality