Fix type error in default PHP migration by narrowing version to native-supported set

[nightnei]

How AI was used in this PR

AI helped to fix it

Proposed Changes

- Narrow DEFAULT_PHP_VERSION via resolveNativePhpVersion() before passing it to getConfiguredPhpBinaryVersion() in apps/cli/migrations/06-install-bundled-default-php.ts and its test, - No runtime behavior change — the default value ("8.4") is already a native-supported version; this only satisfies the type checker.

Testing Instructions

Code review

[wpmobilebot]

📊 Performance Test Results

Comparing b89e8e4 vs trunk

app-size

Metric	trunk	b89e8e4	Diff	Change
App Size (Mac)	1327.01 MB	1327.01 MB	+0.00 MB	⚪ 0.0%

site-editor

Metric	trunk	b89e8e4	Diff	Change
load	1523 ms	1519 ms	4 ms	⚪ 0.0%

site-startup

Metric	trunk	b89e8e4	Diff	Change
siteCreation	8573 ms	8565 ms	8 ms	⚪ 0.0%
siteStartup	4931 ms	4929 ms	2 ms	⚪ 0.0%

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Results are median values from multiple test runs.

Legend: 🟢 Improvement (faster) | 🔴 Regression (slower) | ⚪ No change (<50ms diff)