Add document layer-tree tool

[bbible3]

This PR adds a dedicated MCP tool that returns the active document’s hierarchical layer structure and fixes root-layer coverage so ungrouped layers are not omitted.

What’s included

1. Added new tool: photoshop_get_document_layer_tree
2. Added ActionManager method to build a recursive tree of:

• groups (type: group, children)
• layers (type: layer)

3. Added explicit ungrouped_layers in the response so document-level (non-grouped) layers are always visible.
4. Kept existing photoshop_get_active_document_info behavior intact for backward compatibility.

Why

get_active_document_info returns flat lists, which does not preserve parent/child relationships.
A dedicated tree endpoint is needed to understand actual group membership.

Response shape (new tool)

• success
• document_name
• layer_tree (recursive nodes)
• ungrouped_layers (top-level non-grouped layers)

Validation

1. Verified photoshop_get_document_layer_tree returns nested groups and child layers.
2. Verified root layers now appear in both layer_tree (top-level layer nodes) and ungrouped_layers.
3. Verified count alignment on test document:

• flat layers count from photoshop_get_active_document_info: 13
• ungrouped_layers count from photoshop_get_document_layer_tree: 13

4. Verified photoshop_get_active_document_info remains unchanged and still returns expected flat layer/group lists.

Files changed

• photoshop_mcp_server/ps_adapter/action_manager.py
• photoshop_mcp_server/tools/session_tools.py