Fix #7513: Fix onBackspace handling of multiline messages with emojis or mentions

[kfc35]

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• I have read the README and Contributor Guidelines
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Contributor checklist:

• My contribution is not related to translations.
• My commits are in nice logical chunks with good commit messages
• My changes are rebased on the latest main branch
• A pnpm run ready run passes successfully (more about tests here)
• My changes are ready to be shipped to users

Description

When onBackspace occurs with the cursor at the beginning of a later line in a multiline message, it now short-circuits out to the default backspace handler. This prevents bespoke handling of emoji and mention deletions to erroneously be executed as described in #7513.

Fixes #7513

Testing Approach

Only manual testing was done on macOS 15.6.1 with a local pnpm start, filled with my production data.

• I tested the scenario described in Multiline text with emoticon doesn't handle backspace correctly #7513 and verified expected behavior (the second new line was backspaced instead)
• I tested an additional scenario with Test 1\n Test 2\n@usernameMention and also verified expected behavior (instead of an extra @ sign being inserted after backspacing, no extra @ was generated)
• I tested macOS option-delete and verified that nothing changed.

I tried writing an automated test-electron test for the CompositionInput component, but got stuck on getting the quillRef to simulate events to. I think writing an automated test would include a refactoring of the component to expose it, or I am not savvy enough to figure it out, so I only went with my manual testing.

[kfc35]

I just rebased my changes on the latest main branch after noticing there was a conflict.
I went through my manual tests and ensured that the solution works as intended.

pnpm run ready seems to no longer be passing successfully, but it seems unrelated to my change — running it on main by itself also currently fails for me locally now.