Full sync: Calculate metadata size for trimming and dynamic chunking based on all item not just value (#43421)

* Calculate metadata size based on all data in the item not just the value

* Bug fix

* changelog

* Test changes

* changelog

* Convert to string to ensure strlen doesn't fail

Committed via a GitHub action: https://github.com/Automattic/jetpack/actions/runs/14993133848

Upstream-Ref: Automattic/jetpack@40dcd10

Author: darssen

composer.json

@@ -11,7 +11,7 @@
 		"automattic/jetpack-status": "^5.1.4",
 		"automattic/jetpack-config": "^3.1.0",
 		"automattic/jetpack-my-jetpack": "^5.14.0",
-		"automattic/jetpack-sync": "^4.12.0"
+		"automattic/jetpack-sync": "^4.13.0-alpha"
 	},
 	"require-dev": {
 		"automattic/jetpack-changelogger": "^6.0.5",