Fix metadata xs:dateTime serialization and GZip compression handshake for C# subscriber interop

[ritchiecarroll]

Summary

Fixes two metadata interop bugs between the STTP Python publisher and the STTP C# subscriber (gsfapi reference). Python's lenient implementations accepted both on a Python↔Python round-trip; the C# subscriber did not.

1. xs:dateTime serialization

The Python publisher serialized timezone-aware UTC datetimes in metadata XML with both a UTC offset and a Z designator (e.g. 2024-01-01T00:00:00.000+00:Z) — an invalid xs:dateTime lexical value that the C# subscriber rejects when parsing metadata via .NET DataSet.ReadXml.

Root cause (DataSet._value_to_xml_text() in src/sttp/data/dataset.py): isoformat() emitted a +00:00 offset, rstrip('0') then corrupted the offset, and a Z was appended unconditionally.

Fix: rewrote xsdformat() as a single canonical formatter emitting a bare xs:dateTime (yyyy-MM-ddTHH:mm:ss[.ffffff], no Z/offset), matching the .NET DataSet reference and round-tripping without a timezone shift. _value_to_xml_text() delegates to it, unifying the two previously divergent datetime serializers.

2. Metadata / signal-index-cache GZip compression handshake

The Python publisher GZip-compressed metadata (and the signal index cache) whenever CompressMetadata / CompressSignalIndexCache was set, ignoring the negotiated GZip compression mode. The .NET DataSubscriber default advertises CompressMetadata without GZip, so it received gzip bytes it does not inflate and failed on the 0x1F magic byte.

Fix (src/sttp/transport/subscriberconnection.py): added should_gzip_compress() and gated both compression sites on the content flag AND CompressionModes.GZIP, matching the C# reference (DataPublisher.SerializeMetadata / SerializeSignalIndexCache). Publisher-only — the Python subscriber already always advertises GZip.

Tests

• Unit / regression: test/test_metadata_datetime_xml.py (bare-datetime output + round-trip) and test/test_metadata_compression.py (compress only when flag and GZip). Full suite: 36 passed.
• Cross-language harness (test/interop/): runs a real Python publisher against the real C# subscriber (gsfapi InteropTest --metadata), by default exercising both compression negotiations with a tz-aware UTC sentinel. Verified end-to-end:
  • fixed → C# parses 2033-03-03T03:33:33.123 in both uncompressed and GZip modes;
  • revert date fix → C# fails '…+00:Z' is not a valid AllXsd value;
  • revert compression fix (--compression plain) → C# fails hexadecimal value 0x1F, is an invalid character.

Reviewer notes

• Companion PR Add --metadata mode to InteropTest sample for metadata date/time and compression interop gsfapi#31 adds the C# InteropTest --metadata [--gzip] harness mode this uses (separate repo).
• The compression fix also covers the signal index cache (same handshake bug, same C# reference gating). Because the metadata harness does not subscribe, that path is verified by the unit tests and by Python↔Python subscription (test_pubsub, which negotiates GZip).

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