collation-generation: resolve mismatch between descriptor and commitments core index (#7104)

## Issue
[[#7107] Core Index Mismatch in Commitments and
Descriptor](https://github.com/paritytech/polkadot-sdk/issues/7107)

## Description
This PR resolves a bug where normal (non-malus) undying collators failed
to generate and submit collations, resulting in the following error:

`ERROR tokio-runtime-worker parachain::collation-generation: Failed to
construct and distribute collation: V2 core index check failed: The core
index in commitments doesn't match the one in descriptor.`

More details about the issue and reproduction steps are described in the
[related issue](https://github.com/paritytech/polkadot-sdk/issues/7107).

## Summary of Fix
- When core selectors are provided in the UMP signals, core indexes will
be chosen using them;
- The fix ensures that functionality remains unchanged for parachains
not using UMP signals;
- Added checks to stop processing if the same core is selected
repeatedly.

## TODO
- [X] Implement the fix;
- [x] Add tests;
- [x] Add PRdoc.