Preserve single-line code spans in fallback docstrings

[SAY-5]

When a docstring falls back to escape_markdown, backslashes are added inside inline code spans. For example, object_hook becomes object\_hook, which displays a literal backslash in hover and completion documentation.

Preserve the contents of same-line code spans with matching backtick delimiters, while keeping the existing escaping and whitespace handling outside them. If a matched pair crosses a line, retain the previous escaping for the entire input so this change does not alter ambiguous multiline Markdown parsing.

Refs #606. This addresses the extra backslashes in code spans; signature wrapping is outside this change.

Validation on Python 3.13 / Jedi 0.20.0:

• The new regression matrix produces 14 assertion failures and 11 passing controls before the fix.
• All 47 tests in test_utils.py, test_hover.py, and test_signature.py pass after the fix.
• A separate real Document/Jedi completion-and-resolve smoke check verifies nonempty Markdown and plaintext documentation.
• Ruff 0.9.4 lint and formatting pass across pylsp and test.

The full platform/plugin suite was not run locally. The completion test module requires optional packages absent from this environment; the separate smoke check does not replace that suite.

OpenAI Codex assisted with the code, regression tests, and this description. The final patch received an independent AI review.