fix: Pass null post ID for local drafts in GutenbergKit

[dcalhoun]

Description

Local draft posts have remotePostId defaulting to 0 (Java's long default), which was passed directly to GutenbergKit's setPostId(). GBK expects null for unsaved posts, but was receiving 0 instead.

This change checks isLocalDraft and passes null to setPostId() for local drafts, ensuring the GBK webview correctly identifies new/unsaved posts.

Testing instructions

Open a new draft post in GutenbergKit:

1. Create a new post using the GutenbergKit (experimental) editor
2. Inspect the webview configuration

• Verify the post ID is null (not 0) in the GBK webview

Open an existing published post in GutenbergKit:

1. Open an existing published post in the GutenbergKit editor

• Verify the post ID is the correct remote post ID (not null)

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[wpmobilebot]

📲 You can test the changes from this Pull Request in WordPress Android by scanning the QR code below to install the corresponding build.

	App Name	WordPress Android
	Flavor	Jalapeno
	Build Type	Debug
	Version	pr22580-cd82adf
	Build Number	1484
	Application ID	org.wordpress.android.prealpha
	Commit	cd82adf
	Installation URL	449t7vds7bue8

Note: Google Login is not supported on these builds.

[wpmobilebot]

📲 You can test the changes from this Pull Request in Jetpack Android by scanning the QR code below to install the corresponding build.

	App Name	Jetpack Android
	Flavor	Jalapeno
	Build Type	Debug
	Version	pr22580-cd82adf
	Build Number	1484
	Application ID	com.jetpack.android.prealpha
	Commit	cd82adf
	Installation URL	0gkgd2rpavdqg

Note: Google Login is not supported on these builds.

[wpmobilebot]

🤖 Build Failure Analysis

This build has failures. Claude has analyzed them - check the build annotations for details.