#16997: fix(telegram): preserve entities during text fragment reassembly

## Summary - When long Telegram messages are split into fragments and reassembled, all formatting entities (bold, italic, code blocks, links, mentions) were explicitly discarded by setting `entities: undefined` on the synthetic message. This could cause bot mentions in entities to be lost, potentially causing messages to be ignored in mention-gated groups. - Extracted a `mergeFragmentEntities()` utility that merges entities from all fragments with correctly adjusted `offset` values, accounting for concatenated text positions and optional separators. - Fixed both the text fragment reassembly path (no separator) and the inbound debounce flush path (`"\n"` separator). ## Test plan - [x] Added unit tests for `mergeFragmentEntities()` covering: single fragments, multi-fragment offset adjustment, newline separator handling, bot mention preservation, interspersed empty-entity fragments, empty text fragments, empty input, and extra entity fields (url, user) - [x] All 10 tests pass - [x] TypeScript type-check passes with no errors - [x] Linter and formatter pass with no issues <!-- greptile_comment --> <h3>Greptile Summary</h3> Fixes a bug where Telegram message entities (bold, italic, mentions, links, etc.) were discarded during text fragment reassembly and inbound debounce flushing. Previously, the synthetic merged message set `entities: undefined`, which caused `hasBotMention()` and `expandTextLinks()` to lose formatting and mention data — potentially causing the bot to ignore messages in mention-gated groups. - Extracts a new `mergeFragmentEntities()` utility in `src/telegram/merge-entities.ts` that correctly accumulates UTF-16 offsets across concatenated fragments, accounting for optional separators and empty fragments. - Both call sites in `bot-handlers.ts` (debounce flush with `"\n"` separator, and text fragment flush with no separator) are updated consistently, with the debounce path correctly pre-filtering to non-empty fragments. - Comprehensive test suite covers offset adjustment, separator handling, empty fragments, extra entity fields, and bot mention preservation. <h3>Confidence Score: 5/5</h3> - This PR is safe to merge — it fixes a real entity-loss bug with correct offset math and thorough tests. - The change is well-scoped: a single new utility with clean offset arithmetic, two consistent call sites, and 10 unit tests covering edge cases. The merging logic correctly uses JavaScript string.length (UTF-16 code units) which matches Telegram's entity offset convention. Both call sites maintain consistency between how combinedText is built and how entities are merged. No regressions are introduced — the previous behavior was strictly worse (discarding all entities). - No files require special attention. _{Last reviewed commit: 04a039b} <!-- greptile_other_comments_section --> <!-- /greptile_comment -->