Theme JSON: Avoid rebuilding identical block schemas during sanitization

[mreishus]

Trac ticket: https://core.trac.wordpress.org/ticket/66003

Backport for: WordPress/gutenberg#82203

Use of AI Tools

AI assistance: Yes
Tool(s): ChatGPT
Model(s): GPT-5.6 Sol Pro
Used for: Brainstorming regression causes and test creation

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