fix(webhook-telegram): escape MarkdownV2 code blocks

[dicnunz]

Summary

• escape backticks and backslashes inside Telegram MarkdownV2 preformatted blocks
• add a regression test covering JSON data with both characters

Telegram requires every backtick and backslash inside pre and code entities to be escaped: https://core.telegram.org/bots/api#markdownv2-style

Without this, ordinary payload data such as Windows-style paths or Markdown snippets can break sendMessage parsing.

Verification

• pnpm vitest run packages/webhooks/telegram/src/index.test.ts (9 passed)
• pnpm --filter @profullstack/sh1pt-webhooks-telegram typecheck
• git diff --check

[github-actions]

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