Sample content constrained to 600px MaxHeight in Normal visual state

## Describe the bug

The sample hosting framework applies a hardcoded MaxHeight constraint of 600 pixels to sample content in the "Normal" visual state. This causes sample content to be artificially cut off when the options pane is taller than the sample content area.

## Steps to reproduce

1. Open the Sample Gallery application
2. Navigate to a sample that uses [ToolkitSampleOptionsPane] with a tall options pane (e.g., MarkdownTextBlock Custom Theme Sample)
3. Observe that the sample content area is constrained to 600px height even when the options pane is significantly taller
4. The sample content appears cut off at the bottom

**Problematic code:** [ToolkitSampleRenderer.xaml#L26](https://github.com/CommunityToolkit/Tooling-Windows-Submodule/blob/main/CommunityToolkit.App.Shared/Renderers/ToolkitSampleRenderer.xaml#L26)

## Expected behavior

MaxHeight should be applied to both the sample content and the sample options pane.

Worth also trying a MinHeight instead to allow it to expand without taking all available height.

## Code Platform

- UWP
- WinAppSDK / WinUI 3

## Device form factor

- Desktop

## Additional context

Found during review of https://github.com/CommunityToolkit/Labs-Windows/pull/761#pullrequestreview-3633778180

Öffne CommunityToolkit.App.Shared/Renderers/ToolkitSampleRenderer.xaml in der referenzierten Zeile und führe die Sample Gallery aus. Reproduziere das Problem mit dem MarkdownTextBlock Custom Theme Sample und untersuche den visuellen Zustand Normal zusammen mit seinem hohen Optionsbereich. Die Aufgabe ist abgeschlossen, wenn der Sample-Inhalt nicht künstlich abgeschnitten wird und sein Höhenverhalten dem erwarteten Layout des Optionsbereichs entspricht.

Vom Indexierungsmodell aus dem Issue-Text verfasst.