Enable tree fields in .values() and .values_list() calls

[Copilot]

This PR enables tree fields (tree_depth, tree_path, tree_ordering, and custom tree fields) to be included in .values() and .values_list() calls while maintaining full backward compatibility.

Problem

Previously, tree fields were excluded from .values() calls even when explicitly requested:

# This would not include tree_depth in the result
Node.objects.with_tree_fields().values('name', 'tree_depth')

# Users had to resort to RawSQL workarounds
Node.objects.with_tree_fields().values(
    tree_depth=RawSQL("tree_depth", ()), 
    tree_path=RawSQL("tree_path", ()),
)

Solution

Modified the TreeCompiler.as_sql() method to:

1. Check if tree fields are specifically requested in the .values() call
2. Only include tree fields that were explicitly requested
3. Maintain existing behavior for all other cases

New Functionality

# ✅ Now works - include specific tree fields
Node.objects.with_tree_fields().values('tree_depth')
Node.objects.with_tree_fields().values('name', 'tree_depth', 'tree_path')
Node.objects.with_tree_fields().values_list('tree_depth', flat=True)

# ✅ Works with custom tree fields too
Node.objects.tree_fields(tree_names="name").values('name', 'tree_names')

Backward Compatibility

All existing behavior is preserved:

# Still excludes tree fields (unchanged behavior)
Node.objects.with_tree_fields().values('name')
Node.objects.with_tree_fields().values()

# Still includes all tree fields (unchanged behavior) 
Node.objects.with_tree_fields()

# RawSQL workaround continues to work
Node.objects.with_tree_fields().values(tree_depth=RawSQL("tree_depth", ()))

Implementation Details

The fix required only a 6-line change in TreeCompiler.as_sql():

• Changed the condition from elif self.query.values_select: to elif self.query.values_select is not None:
• Added filtering logic to include only requested tree fields when using .values()
• Comprehensive test suite added covering all scenarios and edge cases

This change works with all supported database backends (PostgreSQL, MySQL, SQLite) since it leverages the existing CTE infrastructure without modification.

Fixes #69.

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