fix: include child table metadata in get_doctype_info

[buildswithpaul]

Summary

get_doctype_info returned only top-level fields, so callers hitting create_document for a parent doctype (Sales Order, Purchase Order, etc.) had no way to discover the child DocType's own fields — and ended up creating the parent with empty child tables. create_document's own description (create_document.py:44) already tells the model "First use get_doctype_info to understand the DocType structure, identify required fields and child tables" — so the gap left the documented workflow broken.

This PR:

1. Adds a child_tables list to the response — one entry per Table / Table MultiSelect field, including each child DocType's full field metadata (same shape as the parent fields array). Callers now have everything they need in a single tool call.
2. Fixes is_single, which was reading meta.istable (child-table flag) instead of meta.issingle. Adds a separate is_child_table key so the two concepts stop being conflated.

Response shape (additions only)

{
  "is_single": false,
  "is_child_table": false,
  "child_tables": [
    {
      "fieldname": "items",
      "label": "Items",
      "fieldtype": "Table",
      "options": "Sales Order Item",
      "reqd": 1,
      "fields": [
        {"fieldname": "item_code", "fieldtype": "Link", "options": "Item", "reqd": 1, ...},
        {"fieldname": "qty", "fieldtype": "Float", "reqd": 1, ...},
        ...
      ]
    },
    ...
  ]
}

Type of Change

• Bug fix
• Feature

Related Issues

Fixes #192

Test Plan

• Added test_get_doctype_metadata_includes_child_tables: asserts User.roles is present in child_tables with recursive field metadata (role field exposed inside).
• Added test_get_doctype_metadata_distinguishes_single_from_child_table: System Settings returns is_single=True, is_child_table=False; Has Role returns is_single=False, is_child_table=True.
• bench --site <site> run-tests --app frappe_assistant_core --module frappe_assistant_core.tests.test_metadata_tools — 21 tests pass (15 pre-existing placeholders skipped).
• Manual sanity check via console against Sales Order: 6 child tables returned with full nested field metadata.

Checklist

• I have targeted the develop branch (not main)
• My code follows the existing code style
• I have tested my changes locally
• I have added/updated tests for the change