Multipolygon recording — missing part validation warning when adding a new part with insufficient points

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**Description**
When recording a multipolygon/multi-lines feature, the app correctly enforces a minimum of 3 points per part for multipolygon and a minimum of 2 points per part for multi-lines with a warning messages:
*"You need to add at least 3 points to every part."* / *"You need to add at least 2 points to every part."*
However, this validation is not triggered consistently during the digitising workflow. When a user adds a new part via **More options → Add part**, adds only 1 point, and then immediately proceeds to add another new part, the warning message is **not displayed**. The user is able to continue digitising without any indication that the previous part is invalid. The error is only shown at the final step when the user attempts to save/record the feature — by which point multiple invalid parts may have been created.

**Environment**
[x] Staging

**Application (+ app version, build, operating system)**
Mergin Maps Mobile App
- iOS version: 2025.6.0 — iOS 26.0
- Android version: 2025.6.0 — Samsung Galaxy A53 5G, Android 15

**Steps to reproduce**
1. Open a project with a multipolygon/multines layer and start recording a new feature.
2. Add the first part with at least 3 points for multipolygons or at least 2 points for multi-lines (valid part).
3. Tap **More options → Add part**.
4. Add only **1 point** to the new part.
5. Tap **More options → Add part** again to start another new part — observe whether the warning message appears.
6. Continue digitising and attempt to save/record the feature.

**Actual results**
No warning message is displayed when switching from an invalid part (with fewer than 3 points) to a new part via **Add part**. The user can continue digitising without any feedback. The error is only shown when attempting to save the entire feature at the end.

**Expected results**
The validation warning *"You need to add at least 3 points to every part"* should be displayed **immediately** when the user attempts to add a new part while the current part has fewer than 3 points. This prevents the user from creating multiple invalid parts unknowingly and provides earlier, more actionable feedback during the digitising workflow.

**Screenshots or logs**

https://github.com/user-attachments/assets/13185cc7-f57b-494d-ba45-0a2634ab3a58

*Conclusion*
*Created by Claude and supervised by a human test analyst.*

Reproduce the workflow in the Mergin Maps mobile app using a multipolygon or multi-lines layer, focusing on More options → Add part after adding too few points. Trace the Add part validation path and ensure the appropriate minimum-points warning appears before another part is created; verify both geometry types on iOS and Android.

Written by the indexing model from the issue text.