refactor(dav): readability improvements

[joshtrichards]

This PR introduces less implementation changes than it may appear. Most of the diff is due to formatting changes, variable renaming, and factoring existing code into new private methods.

The overall goal:
Refactor code for better readability and future maintainability.

Key areas of change

• Code formatting improvements
• Variable renaming for clarity
• Addition and clean-up of comments
• Refactoring lengthy callbacks into private methods
• Minor modernization and readability tweaks (no fundamental logic changes)

Details by Component

RootCollection

• Added class-level docblock
• Updated outdated docblocks
• Improved error messaging

PropFindPlugin

• Added class-level docblock
• Made DI properties readonly
• Moved user session property initialization from constructor to initialize() for Sabre plugin consistency
• Refactored MOUNT_POINT_PROPERTYNAME logic into a private method
• Added inline comments

GroupFoldersHome

• Added class-level docblock
• Improved consistency and usefulness of error messages
• Added parameter type hints
• Ordered private methods after public/interface methods
• Reformatted code
• Added docblocks to private methods

GroupFolderNode

• Added class-level docblock

ACLPlugin

• Added class-level docblock
• Renamed variables for clarity
• Added inline comments to clarify logic
• Ordered private methods after public/interface methods
• Reformatted code
• Added docblocks to property request/update handler implementations
• Moved lengthy property-specific handlers to private methods
• Added docblocks to private methods

Other:

• Updated stubs to include the getDirectDownloadById() method recently added in Server, fixing failing tests

TODO:

• Split out updated stubs into separate PR
• Drop stubs once chore: fix failing tests / add getDirectDownloadById() to stubs  #4235 is merged
• Squash/clean-up commits before merge