add keywords Get Children, Get Children Locators

[zhajjhassan]

Description
The Get Children keyword always returned an empty list because AdapterFactoryImpl.get_children_adapters() was a stub (# TODO return []). This change implements the method using Finder.FindNodes() to resolve all direct children via XPath, following the same pattern as the existing get_adapter() method which uses FindSingleNode() for single element lookups.

ElementNode.RuntimeId was throwing NotImplementedException, which caused a crash when creating child adapters since RuntimeId is read to build unique locators. It now delegates to Helper.GetRuntimeId() which was already available but not wired up.

Additionally, a new Get Children Locators keyword is introduced. While Get Children returns context objects, Get Children Locators returns a list of full XPath locator strings built from the parent path plus each child's role and AutomationId or Name. These strings are human-readable in test logs and can be passed directly to other keywords like Activate or Get Property Value.

Type of Change
Bug fix (non-breaking change which fixes an issue)
New feature (non-breaking change which adds functionality)

How Has This Been Tested?

Test Click All Calculator Digits Via Children

• Uses Get Children to find digit buttons in the calculator's Zehnertastatur group, filters by AutomationId, and activates each one.
  Test Test Get Children
• Verifies Get Children returns 11 child elements (digits 0-9 plus decimal separator) and reads their properties.
  Test Test Get Children Locators
• Verifies Get Children Locators returns 11 XPath strings and that each string can be used with Get Property Value to read the AutomationId.

Test Configuration:
OS: Windows 11 ARM64
Python: 3.13.13
Robot Framework: 7.4.2
.NET Runtime: CoreCLR
Test application: Windows Calculator (Rechner)

Checklist

• My code follows the style guidelines of this project.
• I have performed a self-review of my own code.
• I have commented my code, particularly in hard-to-understand areas.
• I have made corresponding changes to the documentation.
• My changes generate no new warnings.
• I have added tests that prove my fix is effective or that my feature works.
• New and existing unit tests pass locally with my changes.
• Any dependent changes have been merged and published in downstream modules.

Additional Notes
The C# project PlatynUI.Extension.Win32.UiAutomation must be rebuilt after pulling this change so the updated ElementNode.RuntimeId implementation is included in the DLL. Run dotnet build src/PlatynUI.Spy/PlatynUI.Spy.csproj to rebuild all dependent assemblies.