fix(ci): restore iOS 26 test leg by guarding empty matrix.macos at call site

[wmathurin]

Problem

When a matrix include entry omits macos, ${{ matrix.macos }} evaluates to an empty string. When passed as a workflow_call input, an empty string overrides the reusable workflow's default: macos-latest — leaving runs-on: "", which causes GitHub to silently produce no job for that matrix leg. The iOS 26 / Xcode 26 leg has been silently absent from every PR and nightly run since the matrix was restructured.

Credit to @james-enperso for identifying the regression and confirming it against the live GitHub jobs API (PR #474).

Root cause

The issue is at the call site, not in the matrix. ${{ matrix.macos }} passes an explicit empty string, which GitHub Actions does not treat as "not supplied" — so the default: in reusable-workflow.yaml never fires.

Fix

Add || 'macos-latest' at the call site in both pr.yaml and nightly.yaml:

# before
macos: ${{ matrix.macos }}

# after
macos: ${{ matrix.macos || 'macos-latest' }}

This means:

• Any matrix row that sets macos explicitly (e.g. macos: macos-15 on the ^18 leg) passes that value through unchanged.
• Any matrix row that omits macos (e.g. the ^26 leg) automatically gets macos-latest — no per-row bookkeeping required.
• macos-latest remains the single source of truth in reusable-workflow.yaml; future matrix additions won't silently skip if they forget to set macos.

Why not add macos: macos-latest to the ^26 include entry?

That approach (taken in PR #474) works for today's matrix but duplicates the default string across three files and leaves the codebase one forgotten entry away from the same silent-skip bug recurring. The call-site fallback is self-enforcing.

Test plan

• Confirm the next PR run contains the iOS ^26 job again (4 jobs total: permission-check + android-pr + ios-pr (^26) + ios-pr (^18)).
• Confirm ios-pr (^18) still runs on macos-15 (the explicit override is preserved).

🤖 Generated with Claude Code

[github-actions]

	Tests	Passed ✅	Skipped	Failed
iOS ^18 Test Results	35 ran	35 ✅

Test	Result
No test annotations available

[brandonpage]

NIT: I think the correct way to do this is to put macos: macos-latest under matrix. That way it will be used for all unless an include overrides it (the default).