#9343 - Refactor: Resolve bunch of sonar problems 2

[gitquil07]

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

(Screenshots, videos, or GIFs, if applicable)

SonarQube Violation Fixes — Pull Request Description

---

Summary

This PR fixes 5 SonarQube rule violations across 33 files to improve code quality and type safety.

Rule	Description	Files	Fix
S1444	Public static fields should be read-only	2	private static or readonly
S1871	Identical branches in conditional structure	6	Merged duplicate switch cases
S6861	Mutable variables should not be exported	1	export let → export const
S2933	Constructor-only fields should be readonly	20	Added readonly to operation classes
S1121	Assignments in sub-expressions	4	Extracted assignments from conditions/returns

Total: 33 files updated. No functional changes; behavior preserved.

---

How did you fix the issue?

• S1444: Made singleton _instance private in SequenceEventDelegationManager; added readonly to React contextType in Save.tsx.
• S1871: Merged identical switch branches (e.g., case Chem + default, case 'arrow' + case 'plus') to remove redundancy.
• S6861: Replaced export let editorEvents with export const editorEvents and inline initialization; resetEditorEvents() now mutates in place.
• S2933: Added readonly to constructor-only fields in 20 operation classes under packages/ketcher-core/src/application/editor/operations/.
• S1121: Moved assignments out of while conditions and arrow-function returns into separate statements.

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