docs: replace RST double-backtick notation with single backticks in source docstrings

[planetf1]

Context

Follows from #656 (build-time fix for broken cross-reference links).

The root cause is RST-style ````Symbol```` notation in docstrings across the codebase. The build pipeline now auto-corrects this at generation time, but the source should be cleaned up to use standard Markdown/Google-style single backticks.

Scope

91 files, 992 occurrences in `mellea/` and `cli/`.

Detection: run the validator with `--source-dir mellea` — reports all occurrences as warnings.

uv run python tooling/docs-autogen/validate.py docs/docs/api \
    --source-dir mellea --skip-coverage

Example (before/after):

```python

Before (RST)

"""FormatterBackend extends the abstract Backend with a ChatFormatter."""

After (Markdown/Google)

"""FormatterBackend extends the abstract Backend with a ChatFormatter."""
```

Task

Do a bulk find-and-replace of ````word```` → ``word`` throughout `mellea/` and `cli/`. Verify the validator reports 0 warnings afterward.

When complete, the normalize_rst_backticks() pass in decorate_api_mdx.py can optionally be removed (or left as a safety net).