Thinking Path

• Hermes WebUI renders fenced code blocks through repeated post-render passes during normal chat updates.
• For code blocks with a language header, the copy button is appended to the sibling .pre-header, not inside the <pre> element.
• The existing duplicate guard only checked inside <pre>, so repeated addCopyButtons() calls could append duplicate header copy buttons.
• This PR makes the guard check the header before creating a new button.

What Changed

• Detect an existing .code-copy-btn in the .pre-header sibling before creating a new copy button.
• Keep the existing no-header behavior unchanged.
• Added a regression test that verifies the header guard runs before document.createElement('button').

Why It Matters

• Prevents duplicate copy buttons on language-header code blocks after repeated render/cache/streaming update passes.
• Keeps the fix small and localized to the existing vanilla JS helper.

Verification

• python -m pytest tests/test_issue1096_copy_buttons.py -q
• Result: 10 passed

Risks / Follow-ups

• Low risk; the change only broadens the idempotency guard before creating a copy button.
• No layout changes beyond preventing duplicate buttons.

Model Used

• AI-assisted with OpenAI GPT-5.5 via Hermes Agent, using local git/pytest/GitHub CLI tooling.