fix: provide right response when there are failures in multi select changeset mode

[raghavsk]

1. When a change set request fails due to an error in processing one of it's parts, we exit the scenario and don't execute the remaining parts of the change set.
2. We correct the behaviour to return a single error on change set failure.
3. The function handlePart is refactored into:
   • ChangeSet case: getChangeSetResponse getPartRequest get412ErrorInfo* getPartResponse getCombined412ErrorResponse
   • Isolated case: getPartRequest getPartResponse

[changeset-bot[bot]]

🦋 Changeset detected

Latest commit: f48d4034b907619f9e8d2884d200edfbea00176c

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 2 packages

| Name | Type | | ------------------------------------ | ----- | | @sap-ux/fe-mockserver-core | Patch | | @sap-ux/ui5-middleware-fe-mockserver | Patch |

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[cla-assistant[bot]]

All committers have signed the CLA.

[raghavsk]

We shall use the pull request https://github.com/SAP/open-ux-odata/pull/869 for this change.