Raise a clear error for unsupported ROAPI partition column types

[SID-6921]

Summary

`py_type_to_roapi` (src/datarepo/export/roapi.py) maps a partition column's Python value type to a ROAPI data type via a bare dict subscript:

```python
return {int: "Int64", str: "Utf8", bool: "Boolean", float: "Float64"}[py_type]
```

Any `docs_filters` value whose type isn't one of those four (e.g. a `datetime.date` partition column, which is a very ordinary thing to partition by) blows up `export_to_roapi_table` with an opaque `KeyError: <class 'datetime.date'>`, instead of a message that says what's actually unsupported. The rest of this codebase's filter/operator conversion code (`_filter_to_expr`, `filter_to_sql_expr`, `ClickHouseTable._build_query`) consistently raises a `ValueError` naming the bad input — this was the one spot still doing a raw subscript.

Changes

• `py_type_to_roapi` now raises `ValueError(f"Unsupported partition column type {py_type!r} for ROAPI export. Supported types: [...]")` for anything outside the four supported types.
• No change in behavior for the four supported types.

Test plan

• New test fails on `main` with `KeyError: <class 'datetime.date'>` (verified by reverting the source change locally and re-running)
• New tests pass with the fix, including a parametrized check that all four previously-supported types are unaffected
• Full `test/export/` suite passes (18 passed)
• `black --check` clean on both changed files

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[neuralink-code-review-bot]

No material issues. The change replaces an opaque KeyError with a ValueError that names the unsupported type, matching the rest of the filter/export conversion code, and the four previously supported mappings are unchanged. Coverage includes both the helper and the export_to_roapi_table path for a date partition value.

Posted by the code review bot. This is an automated review, not a maintainer approval.