fix(telegram): prevent truncation of valid URLs with entity-marker characters in MarkdownV2 links

[merrcury]

Summary

Fixes @chat-adapter/telegram silently truncating valid MarkdownV2 messages whose link URLs contain an odd number of entity-marker characters (_, *, ~).

trimToMarkdownV2SafeBoundary counted unescaped markers anywhere outside code spans and treated an odd total as an unterminated entity. But per the MarkdownV2 spec, only ) and \ are special inside the (...) URL part of an inline link — so a message ending with e.g. [Read more](https://example.com/page?utm_campaign=a&utm_source=b&utm_channel=c) (3 raw underscores in the URL) was sliced mid-URL, rejected by Telegram with can't parse entities: Can't find end of a URL, and degraded to plain text by the markdown fallback — links and formatting silently stripped.

Changes in findUnescapedPositionsOutsideCode:

• Tracks a link-URL state alongside the existing inFence/inInline tracking: it opens when an unescaped ]( is consumed outside code and closes at the first unescaped ). Markers inside that span are never recorded.
• A link's ] only counts toward bracket pairing once its URL closes, so hard truncation (4096-char limit) that slices mid-URL now leaves the [ unmatched and trims back to before the link — previously the cut left an unterminated ( that Telegram rejected.

The defensive under-limit safety pass from #446 (streaming chunks) is intentionally kept; it's now link-aware.

Test plan

• Added unit tests in packages/adapter-telegram/src/markdown.test.ts:
  • URLs with 1 and 3 (odd) underscores pass through unchanged
  • URL with raw underscores alongside surrounding *bold* entities — unchanged
  • URL with * and ~ in the query string — unchanged
  • Full-pipeline regression: renderPostable + truncateForTelegram on a message ending with a multi-query-param link
  • Hard truncation inside a link URL trims back to before the [
  • Hard truncation inside *bold* still trims at the orphan * (existing behavior)
  • Unpaired _ outside a link URL is still stripped (under-limit pass from fix(telegram): apply MarkdownV2 entity safety trim to streaming chunks #446 preserved)
• All 216 @chat-adapter/telegram tests pass
• pnpm validate passes (knip, lint, typecheck, test, build)

Checklist

• All commits are signed and verified
• pnpm validate passes
• Changeset added (or N/A — see CONTRIBUTING.md) — .changeset/loud-falcons-wave.md, patch for @chat-adapter/telegram
• Documentation updated (or N/A) — N/A, internal truncation behavior, no user-facing API or docs change

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