Don't orphan SplitContainer content when the assignment is invalid

[Dev9269]

SplitContainer.content setter detached the currently-shown widgets before validating the new value:

for old_content in self._content:
    if old_content is not None:
        old_content.app = None
        old_content.window = None
try:
    if len(content) != 2:
        raise TypeError()
    ...

If the assignment was invalid (wrong length, duplicate widget, wrong element type), the exception propagated but the old content had already been orphaned — app/window were reset to None, so the previously-visible widgets detached from the widget tree even though content still reported the old value.

This PR moves the detach to after the new content is fully validated, so a failed assignment leaves the current content attached and intact.

Fixes #4631

PR Checklist:

• I will abide by the BeeWare Code of Conduct
• I have read and have followed the CONTRIBUTING.md file
• This PR was generated or assisted using an AI tool

Assisted-by: Claude Opus 4.5

[freakboy3742]

Good catch - thanks for the report and PR.

However - this has clearly broken CI. I'm guessing this is because the test app lifecycle isn't being honoured because you're manually constructing the SplitContainer widget rather than using the fixture.