fix: ignore malformed custom emoji entities

[Luckylos]

Summary

• Ignore malformed Telegram custom_emoji entities whose offset is outside the source text.
• Preserve the original text and continue rendering instead of throwing from buildStyledChars.
• Keep one bounded diagnostic warning with entity metadata; do not log the message body.

Motivation

A malformed custom-emoji entity currently dereferences styledChars[entity.offset] without checking the offset. This aborts text preparation and can discard an otherwise valid quote. The guard is limited to malformed entity input; valid custom-emoji entities keep the existing behavior.

Tests

• node test-fixes.js
• npm test
• npm run test:visual — all 24 real rendering scenarios passed
• npx eslint test-fixes.js utils/quote-generate/text-prepare.js
• git diff --check

The repository-wide lint command still reports 35 pre-existing errors in unchanged files; the changed files pass targeted lint.