AI Assistant: Update usage of usePostContent for only when necessary (#42149)

* make url optional on useAutosaveAndRedirect as it is only used by one function

* refactor usePostContent hook to expose isEditedPostEmpty

* refactor uses of usePostContent hook

* fix some deprecation warnings

* changelog

Committed via a GitHub action: https://github.com/Automattic/jetpack/actions/runs/13594385339

Upstream-Ref: Automattic/jetpack@e908b00

Author: dhasilva

package.json

@@ -33,7 +33,7 @@
 	"dependencies": {
 		"@automattic/jetpack-analytics": "^0.1.36",
 		"@automattic/jetpack-components": "^0.68.0-alpha",
-		"@automattic/jetpack-shared-extension-utils": "^0.17.3",
+		"@automattic/jetpack-shared-extension-utils": "^0.17.4-alpha",
 		"@wordpress/block-editor": "14.12.0",
 		"@wordpress/blocks": "14.6.0",
 		"@wordpress/compose": "7.17.0",