Fix duplicate rich-editor Tab suggestion inserts

[bartekplus]

Summary

• deduplicate rich-editor Tab accepts so the early bridge and the normal keydown path cannot both accept the same suggestion while the first edit is still pending
• stop forcing an immediate DOM fallback when a generic contenteditable host cancels beforeinput without an immediate mutation, which was duplicating inserts on async hosts like Reddit and Facebook
• add regression coverage for both failure modes in the suggestion session and text-edit service tests

Test Plan

• bun run lint
• bun run format:check
• bun test tests/SuggestionTextEditService.test.ts tests/SuggestionEntrySession.test.ts tests/SuggestionManager.test.ts tests/content_script.behavior.test.ts
• bun run test:e2e
• bun run test:e2e --platform=firefox