Port 'Expose Settings.xml Visibility' from machine

[Copilot]

Ports changes from sillsdev/machine#412 — exposes the <Visibility> field from Paratext Settings.xml.

Changes

• ParatextProjectSettings — Added visibility: Optional[str] = None field
• ParatextProjectSettingsParserBase — Reads <Visibility> element and passes it to the constructor
• Tests — Added test_parse() to both file and zip parser tests asserting all settings fields; updated _create_settings() helper to include "Public" for visibility

parser = FileParatextProjectSettingsParser(project_path)
settings = parser.parse()
assert settings.visibility == "Public"  # read from <Visibility>Public</Visibility> in Settings.xml

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This change is 

[Enkidu93]

@Enkidu93 reviewed 5 files and all commit messages, and made 1 comment.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on ddaspit).

[ddaspit]

@ddaspit reviewed 5 files and all commit messages, and made 1 comment.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on copilot[bot]).

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.69%. Comparing base (9062a9d) to head (6797e82).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #310      +/-   ##
==========================================
+ Coverage   91.68%   91.69%   +0.01%     
==========================================
  Files         359      359              
  Lines       23425    23459      +34     
==========================================
+ Hits        21477    21511      +34     
  Misses       1948     1948              

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