avm2: Don't activate fast-path for non-public Multiname #26530

Job	Run time
Paths filter	6s
Test Node.js 20 / ubuntu-24.04	4m 0s
Build for browser tests	2m 8s
Test Node.js 20 / windows-latest	7m 14s
Test Node.js ${{ matrix.node_version }} / ${{ matrix.os }}	0s
Test Node.js 22 / ubuntu-24.04	3m 58s
Test Node.js 22 / windows-latest	7m 26s
Test in ${{ matrix.browser }}	0s
Test in chrome	1m 41s
Test in firefox	2m 49s
Test in edge	2m 0s
	31m 22s

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