Add --collect-only-tree flag for modern tree-style output

What's the problem this feature will solve?

The current --collect-only output uses an XML-like <Type name> format that feels a bit... vintage:

<Dir pytest>
  <Dir testing>
    <Module test_example.py>
      <Class TestFoo>
        <Function test_bar>

While this format has served us well, it's 2026 now and we have nicer ways to visualize hierarchical data. The angle-bracket format is difficult to scan visually, especially for large test suites, and doesn't leverage modern terminal capabilities.

Describe the solution you'd like

A new --collect-only-tree (or --co-tree) flag that displays collected tests as a proper tree with box-drawing characters:

pytest
└── testing
    └── test_example.py
        └── TestFoo
            └── test_bar

With support for:

• Box-drawing characters (├── └── │), ASCII fallback for non-UTF-8
• Colors: bold for files/dirs, cyan for classes, green for test functions
• Proper tree structure showing sibling relationships

This provides the same information but in a format that's easier to read at a glance.

Alternative Solutions

The existing --collect-only would remain unchanged for backward compatibility and tooling that may parse its output. The tree format is purely additive as a new flag.

One could write a plugin or post-process the output, but this seems useful enough to warrant inclusion in core pytest.

Additional context

I have a working implementation ready: https://github.com/sclaughl/pytest/tree/feature/collect-only-tree-flag

A few design notes:

• Structural vs execution order: The tree shows tests grouped by their hierarchy (directory/module/class), not the order they'll run. The classic --co reflects pytest_collection_modifyitems reordering. --co-tree answers "how are tests organized?" which felt more appropriate for a tree visualization.
• Verbosity: -q shows just summary counts, -v includes docstrings alongside test names
• Architecture: Separated tree-building from rendering so other output formats (JSON tree, etc.) could reuse the structure if desired

Happy to adjust any of this based on feedback. And yes, I checked -- pytest-collect-formatter exists but outputs JSON/YAML/XML for machines, not a visual tree for humans.

Start with the existing --collect-only entry point and the feature/collect-only-tree-flag implementation linked in the issue. Review how pytest_collection_modifyitems affects ordering, then compare the implementation with the requested tree structure, encoding fallback, colors, verbosity, and backward compatibility. Done means the additive flag provides the described output without changing --collect-only.