Move closest helper into core and rewire imports

[Copilot]

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

• Summarized issue: consolidate findClosestAtom/closest utilities into ketcher-core, remove duplicate React copy, and update all call sites.
• Changes:
  • Ported closest.ts and its types into packages/ketcher-core/src/application/editor/shared, exporting via core index (including default closest).
  • Removed legacy core closest.js and the React copy of closest.ts; React now consumes the core implementation and types.
  • Updated React imports (editor, tools, utils) to pull closest/ClosestItem* from core; adjusted context menu type comment.

Example usage after change:

import { closest, ClosestItemWithMap } from 'ketcher-core';

const hit = closest.item(restruct, pos, ['atoms'], skip, options);

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

Original prompt

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This section details on the original issue you should resolve

<issue_title>Change all usages of findClosestAtom function from packages\ketcher-core\src\application\editor\shared\closest.js to usage of findClosestAtom function from packages\ketcher-react\src\script\editor\shared\closest.ts</issue_title>
<issue_description>1. Change all usages of findClosestAtom function from packages\ketcher-core\src\application\editor\shared\closest.js to usage of findClosestAtom function from packages\ketcher-react\src\script\editor\shared\closest.ts
2. Remove packages\ketcher-core\src\application\editor\shared\closest.js file afterwards
3. Rather than having closest.ts in ketcher-react we should move it to ketcher-core and readjust all the import/export expressions related to that</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Change all usages of findClosestAtom function from packages\ketcher-core\src\application\editor\shared\closest.js to usage of findClosestAtom function from packages\ketcher-react\src\script\editor\shared\closest.ts #8939

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