[lvgl] Msgbox fixes and enhancements

[clydebarrow]

What does this implement/fix?

• Apply msgbox styles to the msgbox, not the window shade outer object.
• Add button_style key for customising buttons in msgboxes
• Fix build exception with missing close_button key.

Types of changes

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Other

Related issue or feature (if applicable): fixes

Pull request in esphome-docs with documentation (if applicable): esphome/esphome-docs#4209

Test Environment

• ESP32
• ESP32 IDF
• ESP8266
• RP2040
• BK72xx
• RTL87xx
• host

Example entry for config.yaml:

# Example config.yaml

Checklist:

• The code change is tested and works locally.
• Tests have been added to verify that the new code works (under tests/ folder).

If user exposed functionality or configuration variables are added/changed:

• Documentation added/updated in esphome-docs.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 53.93%. Comparing base (4d8b5ed) to head (736f6bc).
Report is 1277 commits behind head on dev.

Additional details and impacted files

@@            Coverage Diff             @@
##              dev    #7380      +/-   ##
==========================================
+ Coverage   53.70%   53.93%   +0.22%     
==========================================
  Files          50       50              
  Lines        9408     9678     +270     
  Branches     1654     1708      +54     
==========================================
+ Hits         5053     5220     +167     
- Misses       4056     4134      +78     
- Partials      299      324      +25     

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