[Swift 6] Fix mutable state error in FirebaseDataEncoder.swift #1591

Job	Run time
spm-package-resolved	1m 5s
pod-lib-lint (ios, macos-14, Xcode_15.2, 5.9)	6m 52s
pod-lib-lint (ios, macos-15, Xcode_16.2, 5.9)	2m 18s
pod-lib-lint (ios, macos-15, Xcode_16.2, 6)	2m 12s
pod-lib-lint (tvos, macos-14, Xcode_15.2, 5.9)	4m 25s
pod-lib-lint (tvos, macos-15, Xcode_16.2, 5.9)	4m 33s
pod-lib-lint (tvos, macos-15, Xcode_16.2, 6)	2m 27s
pod-lib-lint (macos, macos-14, Xcode_15.2, 5.9)	1m 53s
pod-lib-lint (macos, macos-15, Xcode_16.2, 5.9)	1m 55s
pod-lib-lint (macos, macos-15, Xcode_16.2, 6)	4m 40s
pod-lib-lint (watchos, macos-14, Xcode_15.2, 5.9)	1m 7s
pod-lib-lint (watchos, macos-15, Xcode_16.2, 5.9)	1m 14s
pod-lib-lint (watchos, macos-15, Xcode_16.2, 6)	2m 15s
spm (iOS, macos-14, Xcode_15.2)	2m 18s
spm (iOS, macos-15, Xcode_16.2)	2m 6s
spm (tvOS, macos-14, Xcode_15.2)	4m 48s
spm (tvOS, macos-15, Xcode_16.2)	2m 12s
spm (macOS, macos-14, Xcode_15.2)	2m 41s
spm (macOS, macos-15, Xcode_16.2)	3m 31s
spm (catalyst, macos-14, Xcode_15.2)	2m 33s
spm (catalyst, macos-15, Xcode_16.2)	2m 21s
spm (watchOS, macos-14, Xcode_15.2)	6m 39s
spm (watchOS, macos-15, Xcode_16.2)	2m 37s
	1h 8m 42s

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