fix: treat Compression.NONE sentinel as uncompressed in COPY INTO formats

[simozzy]

Problem

Compression.NONE is a truthy Enum member, so the if compression: guard
in CSVFormat, TSVFormat, NDJSONFormat, and ParquetFormat treats the
no-compression sentinel as "a codec was specified":

if compression:                                   # Compression.NONE is truthy!
    if compression not in [Compression.ZSTD, Compression.SNAPPY]:
        raise TypeError('Compression should be None, ZStd, or Snappy.')
    self.options["COMPRESSION"] = compression.value

• ParquetFormat(compression=Compression.NONE) raises TypeError: Compression should be None, ZStd, or Snappy. — even though NONE is the "no compression" case.
• The CSV/TSV/NDJSON classes don't raise, but emit a spurious COMPRESSION = NONE option for the sentinel.

Fix

Skip the block for both None and the Compression.NONE sentinel, so the
uncompressed case renders no COMPRESSION option:

if compression and compression is not Compression.NONE:
    ...

Applied to all four format classes. Explicit codecs are unchanged, and
ParquetFormat still rejects codecs its writer can't use (gzip/bz2/zip).

Tests

tests/test_copy_format.py — constructor-level (no DB):

• None and Compression.NONE emit no COMPRESSION option, across all four classes.
• Explicit codecs are still emitted where valid.
• ParquetFormat still raises TypeError for unsupported codecs.

Context / downstream

Found in PlaidCloud's data-export path: every Parquet export with the default
(no) compression failed with this TypeError. Tracked there as:

• PlaidCloud/plaid#6305 (story sc-22680) — currently carries a call-site
  workaround; this is the root-cause fix.
• Discovered while building the AI export-step sub-agent (story sc-22516).

Note: this fork's main is at 0.5.1, while PlaidCloud pins PyPI
databend-sqlalchemy==0.5.5. To consume this fix, plaid will either re-pin at
the fork or this should be upstreamed to databendlabs/databend-sqlalchemy.

🤖 Generated with Claude Code

[rad-pat]

Ok apart from the test which does not follow the Sqlalchemy testing suite pattern

[simozzy]

@rad-pat Reworked the test to follow the SQLAlchemy testing-suite pattern — tests/test_copy_format.py is now a fixtures.TestBase + AssertsCompiledSQL class (__only_on__ = "databend") asserting the rendered FILE_FORMAT = (...) via self.assert_compile(...), mirroring test_copy_into.py, with assert_raises_message for the ParquetFormat rejection. The old direct .options-dict pytest checks are gone. Verified the compiled output offline against the databend dialect.

[simozzy]

@rad-pat ticket logged with databend/sqlaclhemy databendlabs#75