fix(chat): increase z-index of chat editing overlay to appear above feedback overlay (#306360)

[speedsters]

Summary

Fixes issue #306360 where the "Keep" action in the chat editing overlay appears underneath the "Submit Feedback" button in the agent feedback overlay.

Problem

When both the chat editing overlay (with Keep/Undo actions) and agent feedback overlay (with Submit Feedback) are visible, they share the same z-index (100) and position. This causes the "Keep" action to appear underneath the feedback submit button, making it inaccessible to users.

Root Cause

Both overlays had the same z-index value (100):

• ChatEditingOverlayController: z-index 100
• AgentFeedbackOverlayController: z-index 100

Fix

Increased the z-index of ChatEditingOverlayController from 100 to 101 so it renders on top of the AgentFeedbackOverlayController.

Testing

All z-index values verified:

• ChatEditingEditorOverlay.ts: z-index = 101 ✓
• AgentFeedbackEditorOverlay.ts: z-index = 100 ✓

Fixes #306360

[vs-code-engineering]

📬 CODENOTIFY

The following users are being notified based on files changed in this PR:

@jrieken

Matched files:

• src/vs/workbench/contrib/chat/browser/chatEditing/chatEditingEditorOverlay.ts

[speedsters]

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