7083 Delete operation on Sequence mode causes hanging bonds and undeletable monomers

[KristinaKasyanovskaya]

How the feature works? / How did you fix the issue?

(Screenshots, videos, or GIFs, if applicable)

Check list

• unit-tests written
• e2e-tests written
• documentation updated
• PR name follows the pattern #1234 – issue name
• branch name doesn't contain '#'
• PR is linked with the issue
• base branch (master or release/xx) is correct
• task status changed to "Code review"
• reviewers are notified about the pull request

[AlexeyGirin]

@copilot resolve the merge conflicts in this pull request

[AlexeyGirin]

@copilot resolve the merge conflicts in this pull request

[AlexeyGirin]

@copilot resolve the merge conflicts in this pull request

[AlexeyGirin]

@copilot resolve the merge conflicts in this pull request

[AlexeyGirin]

@copilot resolve the merge conflicts in this pull request

[Copilot]

@copilot resolve the merge conflicts in this pull request

Resolved the merge conflicts in packages/ketcher-core/src/application/editor/modes/SequenceMode.ts (commit cdb634a). The only conflicting file combined master's refactor (the monomersBeingDeleted handling, nullish-coalescing, named sensePhosphateAdditionallyDeleted/antisensePhosphateAdditionallyDeleted booleans, and the !isEditMode guard for selection deletion) with this PR's fix (the backbone delete "-" rewrite using firstConnected/secondConnected polymer-bond detection and the unified deleteHandler). The now-unused previouseNodeInBackbone variable was removed. ketcher-core typecheck and build pass.