project_panel: Select the whole folder name when renaming

[IbrahimKhan12]

Objective

• Renaming a folder whose name contains a dot pre-selects only the text before
  the last dot, treating the suffix as if it were a file extension.
• Fixes Renaming folder doesn't highlight name after last dot #59294

Solution

• ProjectPanel::rename_impl computed the rename editor's initial selection with
  Path::file_stem().len() for every entry. For a directory such as my.folder,
  file_stem() returns my, so only my was selected.
• Directories have no extension, so select the whole name for them. Files keep
  the existing behavior (the last extension is left unselected for quick renames).

Testing

• Added test_rename_folder_with_dot_selects_whole_name in project_panel_tests.rs.
  It fails before the change (folder selection ends at offset 2 of my.folder) and
  passes after (offset 9, the whole name); it also asserts a file still leaves the
  last extension unselected.
• cargo test -p project_panel — all 100 tests pass.
• cargo clippy -p project_panel --all-targets --all-features -- --deny warnings and
  cargo fmt -p project_panel -- --check are clean.

Self-Review Checklist:

• I've reviewed my own diff for quality, security, and reliability
• Unsafe blocks (if any) have justifying comments
• The content adheres to Zed's UI standards (UX/UI and icon guidelines)
• Tests cover the new/changed behavior
• Performance impact has been considered and is acceptable

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Release Notes:

• Fixed renaming a folder with a dot in its name selecting only the part before the dot.

[MrSubidubi]

Nice fix, thanks for this and congrats to your first contribution! 🎉