Defensive hardening improvements for 6.9.3

[priethor]

This PR brings trunk in sync with the defensive hardening improvements released in SCF 6.9.3.

• Harden REST field value formatting
• Harden block template resolution
• Harden relational field query results
• Harden Options Page save scoping
• Harden multisite Local JSON write scoping
• Sync the 6.9.3 changelog, generated documentation, and version metadata

Use of AI Tools

Codex and Claude were used to implement these changes, guided and reviewed by @ockham and me.

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Props priethor, bernhard-reiter.

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[priethor]

The failing e2e tests are also occurring in trunk due to an unrelated WordPress trunk change; I'll merge this with only those tests failing and follow up with a patch to fix them.