chat customizations: fix clipping (#304281)

* fix: re-layout list when toggling browse mode in MCP/plugin widgets

When toggling browse mode, the back link appears/disappears but
layout() was never re-called, causing the list height to not
account for the changed chrome. This clipped the last entry.

Cache the last layout dimensions and re-call layout() after
toggling browse mode.

Fixes https://github.com/microsoft/vscode/issues/304139

* fix: register IProductService in AI customization component fixtures

The AICustomizationListWidget recently added a dependency on
IProductService but the component fixtures were not updated,
causing all AI customization fixtures to fail with
'depends on UNKNOWN service productService'.

* fix: address PR review — remove manual layout() in fixtures, fix null! in ParsedPromptFile