fix(ios): use parent padding in flexbox child measure specs - #11469

PR Checklist

• The PR title follows our guidelines: https://github.com/NativeScript/NativeScript/blob/master/tools/notes/CONTRIBUTING.md#commit-messages.
• There is an issue for the bug/feature this PR is for. (See below — likely related: Cannot set label vertical padding on iOS #5194, Label in FlexboxLayout isn't sized correctly on iOS #4834)
• You have signed the CLA.
• All existing tests are passing.
• Tests for the changes are included — see testing note below.

What is the current behavior?

On iOS, FlexboxLayout.getChildMeasureSpec is called with the child's own padding where the AOSP FlexboxLayout implementation it's ported from passes the parent's padding plus the child's margins:

// AOSP / correct — already used at index.ios.ts:388-389:
getChildMeasureSpec(spec, this.effectivePadding* + lp.effectiveMargin*, ...)

// Bug — used at three other call sites:
getChildMeasureSpec(spec, lp.effectivePadding* + lp.effectiveMargin*, ...)   // child's padding!

The mismatch is visible whenever a flex child is re-measured against a fixed parent constraint. Concretely: a ROW child inside a COLUMN flexbox that gets stretched (_stretchViewVertically re-measure) is re-measured with EXACTLY <rowHeight> on the cross axis, and the child-spec computation subtracts the child's own padding — so a control with 10dip vertical padding inside a 37dip row is clamped to 17dip content height and its label collapses.

This shows up as buttons/labels squished to a minimum height inside items-center flex rows — plausibly the root cause behind #5194 (padding ignored inside flexbox-layout) and possibly #4834.

What is the new behavior?

All getChildMeasureSpec call sites in index.ios.ts now consistently subtract the parent's padding and the child's margins, matching the AOSP reference and the two call sites that were already correct (lines 388–389).

Verified on a real app: padded buttons inside stretched rows measure 37dip (correct) instead of 17dip (collapsed).

Android is unaffected — it delegates to org.nativescript.widgets.FlexboxLayout (the real Google Flexbox library).

Testing note

The affected code runs inside the iOS measure pass; packages/core has no flexbox spec today and Vitest mocks the platform globals, so a unit test would need to mock the whole native measure pipeline. I verified the fix by patching the installed package in an app: controls that previously measured 85x17 now measure 85x37 with visible labels. An apps/automated case asserting child height inside an items-center row would cover this going forward — happy to add one if you'd like.