Propagate serialize_kwargs to ChatMessage

[ahuang11]

Closes #7143

import panel as pn

pn.extension()

ci = pn.chat.ChatInterface()
ci.send("Hello")
ci.add_step("Hello", "World")
ci.serialize(
    prefix_with_container_label=False,
    prefix_with_viewable_label=False
)

[{'role': 'user', 'content': 'Hello'},
 {'role': 'user', 'content': '((Hello))'}]

[codecov]

Codecov Report

Attention: Patch coverage is 83.33333% with 1 line in your changes missing coverage. Please review.

Project coverage is 82.19%. Comparing base (0b76e99) to head (1d6f4cc).
Report is 2 commits behind head on main.

Files	Patch %	Lines
panel/chat/interface.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7146      +/-   ##
==========================================
- Coverage   82.19%   82.19%   -0.01%     
==========================================
  Files         327      327              
  Lines       48970    48974       +4     
==========================================
+ Hits        40253    40255       +2     
- Misses       8717     8719       +2     

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