fix(csharp): set BUFFER_LENGTH to null in GetColumns result; re-enable CanGetColumnsExtended for SEA (PECO-3008)

[eric-wang-1990]

Problem

CanGetColumnsExtended was skipped for SEA (Skip.If(Protocol == "rest")) because the SEA path returned different BUFFER_LENGTH values than Thrift.

Root cause: FlatColumnsResultBuilder (used by the three-calls fallback path via GetColumnsAsync) called ColumnMetadataHelper.GetBufferLength which returned non-null values for numeric types (BOOLEAN=1, TINYINT=1, SMALLINT=2, INT=4, FLOAT=4, BIGINT=8, DOUBLE=8, TIMESTAMP=8, DATE=8, DECIMAL=computed). The expected test values and the Thrift path both return null for all columns.

The JDBC spec defines BUFFER_LENGTH as an unused column — it should always be null. CreateExtendedColumnsResult (the DESC TABLE EXTENDED path) already returned null unconditionally via bufferLengthBuilder.AppendNull(). FlatColumnsResultBuilder was inconsistent.

Changes

• FlatColumnsResultBuilder: replace the GetBufferLength call with AppendNull(), making BUFFER_LENGTH consistently null across all result paths (matching CreateExtendedColumnsResult and Thrift).
• StatementTests.cs: remove the Skip.If(Protocol == "rest") guard from CanGetColumnsExtended.

Test Plan

CanGetColumnsExtended now runs for both Thrift and SEA protocols, covering both useDescTableExtended=true and useDescTableExtended=false.

Fixes PECO-3008

🤖 Generated with Claude Code

[Copilot]

Pull request overview

This PR aligns the C# driver’s GetColumns/GetColumnsExtended metadata results across SEA (REST) and Thrift by making BUFFER_LENGTH consistently null, and re-enables the previously skipped SEA E2E test that validates GetColumnsExtended.

Changes:

• Update FlatColumnsResultBuilder to always emit null for BUFFER_LENGTH instead of computing numeric byte sizes.
• Re-enable CanGetColumnsExtended for SEA by removing the Protocol == "rest" skip guard.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
csharp/src/FlatColumnsResultBuilder.cs	Makes BUFFER_LENGTH always null to match the Thrift path and the DESC TABLE EXTENDED path.
csharp/test/E2E/StatementTests.cs	Removes the SEA skip so CanGetColumnsExtended runs for REST/SEA again.

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[msrathore-db]

The BUFFER_LENGTH → AppendNull() change is correct and matches the Thrift / DESC TABLE EXTENDED paths. However, I tested this branch live against a SQL warehouse with protocol=rest, and removing Skip.If(Protocol == "rest") does not make CanGetColumnsExtended pass on SEA:

• useDescTableExtended=true (DESC TABLE EXTENDED path) → passes.
• useDescTableExtended=false (the FlatColumnsResultBuilder 3-call path this PR touches) → fails, on fields beyond BUFFER_LENGTH:
  • SQL_DATA_TYPE: actual -6, expected null. FlatColumnsResultBuilder.cs:92 does sqlDataTypeBuilder.Append(info.ColType[i]), whereas the DESC path (DatabricksStatement.cs:1089) and Thrift build it all-null — same class of issue as BUFFER_LENGTH, one field over.
  • IS_NULLABLE: PK column c_int returns "NO" on SEA's 3-call path vs the expected "YES" (the PK_IS_NULLABLE:YES placeholder for the false variant).

I confirmed locally that nulling SQL_DATA_TYPE clears the first diff, but IS_NULLABLE still fails afterward — so the BUFFER_LENGTH fix is necessary but not sufficient.

Suggest either (a) reconciling SQL_DATA_TYPE and IS_NULLABLE in the SEA 3-call path before dropping the skip, or (b) scoping the skip to useDescTableExtended=false + SEA only.

Heads-up: if CI doesn't run the SEA matrix for this test, this will merge green while the SEA path it's meant to enable stays broken (the test fails whenever SEA is actually exercised).