Capping Mex sometimes not possible due to nearby building

### Description

In the following clip we can see that a construction turret that was transported near a t1 Mex stops the player from upgrading the Mex to T2 with the selected air con. When the con turret is reclaimed, the t2 Mex can be built over the t1 mex

https://youtu.be/IrVaOtaOdkc?t=21m38s (at 21m 38s)

### Expected Behaviour

A t1 Mex can always be capped with a t2 Mex.

### Actual Behaviour

Buildings placed nearby via transport can sometimes block it.

### Reproduction steps

_No response_

### Other

Also, NotReal should be banned for reclaiming his teammate's structure /s

Start by reviewing the linked gameplay clip at 21:38 and trace the building-placement and Mex-upgrade behavior involved when a transported construction turret is nearby. Reproduce the case if possible, then verify that a T1 Mex can be upgraded to T2 despite the nearby building, while preserving the blocking behavior for genuinely invalid placements.

Written by the indexing model from the issue text.