fix(core): escape Telegram HTML output (#72)

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📝 Description

What: Escape raw HTML control characters in LLM explain output before applying the supported Telegram HTML formatting tags.

Why: Gemini multimodal/text explanations can contain normal comparison text like x < y or raw snippets like <script>, which Telegram rejects when sent with parse_mode=HTML. In the SQS explain path that failure re-raises, causing retries and repeated failed processing or duplicate replies.

Verification:

• PYTHONPATH=src/bot python3 - <<'PY' ... sanitizer sanity checks passed
• python3 -m py_compile src/bot/services/ai/telegram_html.py tests/test_telegram_html.py
• uv run pytest tests/test_telegram_html.py tests/test_explain_multimodal.py could not run: uv is not installed
• python3 -m pytest tests/test_telegram_html.py tests/test_explain_multimodal.py could not run: pytest is not installed

🚀 Type of Change

• 🐛 Bug Fix (Fix an issue)
• ✨ New Feature (Add a new feature)
• 🏗️ Infrastructure (CDK/AWS resource changes)
• ♻️ Refactoring (Code refactoring, no logic changes)
• 📚 Documentation (Documentation update)

🏗️ Infrastructure Changes (Crucial)

• Resources Created: None
• Resources Modified: None
• Resources Deleted: None
• Cost Impact: None

✅ Checklist

• I have performed a self-review of my code.
• I have run pre-commit run --all-files locally and fixed all issues.
• I have added/updated Unit Tests (pytest).
• My changes do not generate new warnings.
• (If Infra change) I have run cdk diff locally to verify changes.

🔗 Related Issues

• Fixes feat(bot): explain multimodal + document auto summary #72