fix(repl): fix multiline editing and completer navigation with keybindings

[notstanzinn]

…dings

Resolves #{{TODO: add issue number}}.

Description

What is the purpose of this pull request?

This pull request:

• Fixes multiline line boundary editing (deleteLeft, moveLeft, moveRight) in the REPL. When keybindings were merged, editor actions intercepted backspace and arrow keys without delegating to MultilineHandler, preventing multiline backspacing into previous lines and moving across lines.
• Adds public boundary methods (moveLeft, moveRight, deleteLeft) to MultilineHandler and delegates to them from EditorActions when at line boundaries.
• Reorders completer navigation in beforeKeypress to precede EditorActions, preventing duplicate character deletions on backspace while navigating completion previews.
• Adds a dedicated integration test suite (test.multiline_editing.js) verifying multiline continuous backspaces, left/right arrow line transitions, and keybinding aliases (ctrl+h).

Related Issues

Does this pull request have any related issues?

This pull request has the following related issues:

• [BUG]: Multiline editing is broken in the REPL #4970
• [RFC]: add support for custom key bindings in the REPL #2647

Questions

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No.

Other

Any other information relevant to this pull request? This may include screenshots, references, and/or implementation notes.

No.

Checklist

Please ensure the following tasks are completed before submitting this pull request.

• Read, understood, and followed the contributing guidelines.

AI Assistance

When authoring the changes proposed in this PR, did you use any kind of AI assistance?

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• No

If you answered "yes" above, how did you use AI assistance?

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Disclosure

If you answered "yes" to using AI assistance, please provide a short disclosure indicating how you used AI assistance. This helps reviewers determine how much scrutiny to apply when reviewing your contribution. Example disclosures: "This PR was written primarily by Claude Code." or "I consulted ChatGPT to understand the codebase, but the proposed changes were fully authored manually by myself.".

-I used Antigravity to research the root cause of the keypress event interception in @stdlib/repl and understand the code base

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