Fix session isolation when prechat fields are absent

[andrea-sdl]

Description

This fixes a regression introduced in the messaging channel session-isolation change. Sites without configured prechat fields were exiting early and never applying restrictSessionOnMessagingChannel, so channels could still share a session unexpectedly.

The root cause was coupling two unrelated behaviors in the same early-return branch: session isolation and prechat field injection. This update keeps prechat field injection conditional, but always applies session isolation when embedded messaging is ready.

Pre-review checklist

Please make sure the items below have been covered before requesting a review:

• This change works and has been tested locally or in Codespaces (or has an appropriate fallback).
• This change has relevant unit tests (if applicable).
• This change has relevant documentation additions / updates (if applicable).
• I've created a changelog description that aligns with the provided examples.

Pre-deploy checklist

• VIP staff: Ensure any alerts added/updated conform to internal standards (see internal documentation).

Steps to Test

1. Configure Agentforce with CMP enabled and no prechat fields configured.
2. Load a page where embedded messaging initializes.
3. Verify channel changes do not reuse a shared session unexpectedly.
4. Repeat with prechat fields configured and verify hidden prechat fields are still applied.

[Copilot]

Pull request overview

Fixes a regression where session isolation for Embedded Messaging wasn’t applied when prechatFields are absent, allowing sessions to be unexpectedly shared across channels.

Changes:

• Removes the early return that prevented restrictSessionOnMessagingChannel from being applied when no prechat fields are configured.
• Makes hidden prechat field injection conditional (hasPrechatFields) while always applying session isolation on onEmbeddedMessagingReady.

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[pandah3]

@andrea-sdl Can you clarify the testing steps? Is this correct?

• Local dev env, have VIP_AGENTFORCE_CONFIGS set up
• Add add_filter( 'vip_agentforce_prechat_fields', '__return_empty_array' ); in a client-mu-plugins file (e.g client-mu-plugins/test-no-prechat.php)
• Set CMP setting to "Custom" in WP Admin
• Browser devtools: window.AgentforceCMP.loadSDK();
• Wait for the SDK to load then check: window.embeddedservice_bootstrap.settings.restrictSessionOnMessagingChannel
• Repeat test without the mu-plugins file

[andrea-sdl]

Local dev env, have VIP_AGENTFORCE_CONFIGS set up
Add add_filter( 'vip_agentforce_prechat_fields', '__return_empty_array' ); in a client-mu-plugins file (e.g client-mu-plugins/test-no-prechat.php)
Set CMP setting to "Custom" in WP Admin
Browser devtools: window.AgentforceCMP.loadSDK();
Wait for the SDK to load then check: window.embeddedservice_bootstrap.settings.restrictSessionOnMessagingChannel
Repeat test without the mu-plugins file

Yes, sorry for not including a full test since this was an edge case.

The testing steps you included look good. To make testing easier I would put the filter inside the env.php instead.
An alternative that requires no extra filter would be to run window.vipAgentforceConsentData?.prechatFields = [] instead of adding the filter. What we want to confirm is that we restrict the messaging channel session if we don't have prechat fields.