[CI/mock_uss] Support additional USS behaviors

[mickmis]

USSs may have differences in their behaviors which are reasonable but might impact the test scenarios by branching into different cases.
Example:

• a USS may remove a planned flight if a modification attempt is rejected
• a USS may not support modification of a flight or an activated flight

Our implementation of the mock USS does not currently have those behaviors. This leads to untested scenario conditions in the CI. Because of that, it is hard to validate that the scenario behaves correctly w.r.t. to a USS, and it is easy for those to break over time.

Is your feature request related to a problem? Please describe.
Recently many issues were reported by a USS that does not support modifications of its activated flights, and addressing those issues required a lot of trial-and-error and back-and-forth. Moreover it is hard to guarantee that there will not be any regression in the future.

Describe the solution you'd like

• Mock USS to support being configured to have different behaviors
• Execute those alternative scenario conditions in the CI

Additional context
This issue has been discussed at the 2025-11-04 InterUSS weekly meeting. As discussed there, mock USS is lacking at the moment structure to properly handle different behaviors.

[the-glu]

@mickmis I started a draft in #1290, feedback welcome :)