Add TOML support to mdtest

[ntBre]

Summary

This PR was split off from #26772 to add support for testing TOML lint rules to mdtest. We use the
toml_parser crate to extract comments and now call lint_pyproject_toml in ruff_mdtest. Because we already use toml blocks for test configuration and because Ruff's current TOML lints need to know the filename, I opted to require a filename for linted TOML blocks. For example,

```toml
lint.select = ["F401"]
```

remains a configuration block, but

`ruff.toml`:

```toml
lint.select = ["F401"]
```

will be linted.

Test Plan

Added one test for RUF200 and unit tests for the parsing itself.

[astral-sh-bot]

Typing conformance results

No changes detected ✅

Current numbers

The percentage of diagnostics emitted that were expected errors held steady at 94.61%. The percentage of expected errors that received a diagnostic held steady at 90.27%. The number of fully passing files held steady at 97/134.

[astral-sh-bot]

Memory usage report

Memory usage unchanged ✅

[astral-sh-bot]

ecosystem-analyzer results

No diagnostic changes detected ✅

Full report with detailed diff (timing results)

[astral-sh-bot]

ruff-ecosystem results

Linter (stable)

✅ ecosystem check detected no linter changes.

Linter (preview)

✅ ecosystem check detected no linter changes.

Formatter (stable)

✅ ecosystem check detected no format changes.

Formatter (preview)

✅ ecosystem check detected no format changes.

[MichaReiser]

Nice, I didn't realize that this would require special casing for comments. But it doesn't seem to be to big a change