Fix Scaffold having status bar when primary is false

[PurplePolyhedron]

When primary is false, Scaffold is not at the top of screen, so it should not have a status bar.
fixes #175062

Pre-launch Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the Tree Hygiene wiki page, which explains my responsibilities.
• I read and followed the Flutter Style Guide, including Features we expect every widget to implement.
• I signed the CLA.
• I listed at least one issue that this PR fixes in the description above.
• I updated/added relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or this PR is test-exempt.
• I followed the breaking change policy and added Data Driven Fixes where supported.
• All existing and new tests are passing.

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[gemini-code-assist]

Code Review

This pull request correctly addresses the issue where a Scaffold with primary: false would incorrectly include a status bar gesture detector. The change is simple and effective, wrapping the status bar logic in a condition that checks widget.primary. The accompanying tests are well-written, covering both the intended behavior and preventing potential regressions. I have one minor suggestion to fix a typo in a test description.

[navaronbracke]

Drive-by nit: This can use an early return instead, to reduce the diff / indent?

if (!widget.primary) {
  break;
}

_addIfNonNull(...);

[PurplePolyhedron]

Good idea! Thanks for the suggestion

[dkwingsmt]

LGTM, thank you!

[dkwingsmt]

Can you also add a paragraph explaining this to the document of [primary]?

[Piinks]

LGTM with a couple of doc nits, thanks for the contribution!

[Piinks]

Suggested change

	/// If ture, on iOS and macOS, tapping the status bar scrolls the app's
	/// If true, on iOS and macOS, tapping the status bar scrolls the app's

[Piinks]

Suggested change

	// On iOS and macOS, tapping the status bar scrolls the app's primary scrollable
	// On iOS and macOS, if `primary` is true, tapping the status bar scrolls the app's primary scrollable

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[Piinks]

LGTM