fix(messages): avoid code block placeholder collisions

[rissrice2105-agent]

Summary

• generate code-block placeholders with a prefix that cannot collide with the original message
• preserve literal text such as __QRYPTCHAT_CODE_BLOCK_0__
• add regression coverage for the collision case

Bug

The formatter used predictable placeholder text while processing fenced code blocks. If a user message contained the same literal marker, the rendered code block replaced that text instead and left the real block as a placeholder.

Tests

• focused URL converter suite: 5/5 passed with a minimal Vitest config
• direct regression assertion: passed
• changed-file oxlint: passed
• git diff --check: passed

The repository's normal Vitest config is currently blocked before test execution by the pre-existing @vitejs/plugin-react / vite/internal package export mismatch. No invoice will be sent unless this PR is merged and accepted.