Edit multipoint feature: Add button disabled – cannot add new point

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**Description**
When editing a multipoint feature, the Add button is disabled, so it’s not possible to add a new point to the feature. This looks like a regression of #2240 (previously fixed).

**Environment**
- Staging

**Application (+ app version, build, operating system)**
- App version: 2025.5.0-rc2
- Android:
- Samsung phone: Android 15
- Samsung tablet: Android 9

**Steps to reproduce**
1. Open a project with a multipoint layer (e.g., tc04_recording).
2. Select an existing multipoint feature.
3. Tap Edit -> Edit geometry.
4. Try to add a new point using the Add button.

**Actual results**
- The Add button is disabled in edit geometry.
- It’s not possible to add a new point to the multipoint feature.

**Expected results**
- The Add button should be enabled when editing multipoint geometry.
- Users should be able to add additional points to an existing multipoint feature.

**Notes**
- Potential regression of closed issue #2240.

**Screenshots or logs**

*Conclusion*
*Created by GitHub Copilot and supervised by a human test analyst.*

Reproduce the issue by opening a project with a multipoint layer such as tc04_recording, selecting an existing feature, and entering Edit geometry. Compare the current behavior with regression reference #2240; done when the Add button is enabled and an additional point can be added to an existing multipoint feature.

Written by the indexing model from the issue text.