fix: protect tilde-fenced code blocks from strikethrough conversion

[blaipr]

Problem

The strikethrough pre-processor in md_to_confluence_storage protected fenced code by splitting on ```.*?``` only. Python-Markdown's fenced_code extension also accepts ~~~ fences and runs longer than three characters, so a ~~ inside a ~~~ block (a shell here-doc, Lua's ~~, Perl) was rewritten to <del>…</del> inside the code, and a block containing a literal ``` ```` was split at the wrong place.

Change

Fences are now found with the same shape the extension uses — ^({3,}|~{3,})opening a line, closed by the identical run at the start of a line — and the text between fences goes through the existing inline-code-aware replacement. The function is split into_replace_strikethrough(fence walk) and_strike_outside_code` (inline spans), with no change to what happens outside fences.

Compatibility

Upload only. Backtick fences and inline code behave exactly as before; ~~~ fences and 4+-character fences are now left untouched like backtick ones.

Checklist

• ruff check src tests, ruff format --check src tests and python -m pytest tests/ pass
• Tests added for the change (tests/test_md_to_storage.py::TestStrikethrough)
• Docs updated (docs/conversion.md, Code blocks)
• docs/conversion.md updated because an equivalence changed
• CHANGELOG.md updated under Unreleased