fix(csharp): canonicalize identifier case in SEA get_primary_keys (PECO-3045) [SEA]

[eric-wang-1990]

What's Changed

The SEA path of GetPrimaryKeysAsync used the user-provided input names
(_metadataCatalogName / _metadataSchemaName / _metadataTableName)
verbatim when building result rows. When callers pass non-canonical case
(e.g. catalog=\"MAIN\" for a server-lowercase catalog main) the
TABLE_CAT / TABLE_SCHEM / TABLE_NAME result columns echo back the
input case instead of the canonical lowercase form. The Thrift path
returns canonical names from the server, and JDBC's SDK path reads
catalogName / namespace / tableName from the SHOW KEYS response —
so SEA was the outlier.

This change makes SEA read those three columns from each SHOW KEYS
record batch and use them when populating result rows, with a defensive
fallback to a lowercased copy of the user input if the columns are
missing or null. It mirrors how FetchCrossReferenceAsync already
reads parentCatalogName / parentNamespace / parentTableName from
SHOW FOREIGN KEYS in the same file.

Why

PECO-3045 — D17: SEA echoes input case in get_primary_keys result
columns; Thrift returns canonical names. JDBC spec says results should
be canonical.

Red → Green proof

Before fix (SEA_GetPrimaryKeys_ReturnsCanonicalCase_InResultColumns):

[xUnit.net 00:00:01.43]     ...SEA_GetPrimaryKeys_ReturnsCanonicalCase_InResultColumns [FAIL]
  Error Message:
   Assert.Equal() Failure: Strings differ
           ↓ (pos 0)
Expected: \"main\"
Actual:   \"MAIN\"
           ↑ (pos 0)
Failed!  - Failed:     1, Passed:     0, Skipped:     0, Total:     1

After fix:

Passed!  - Failed:     0, Passed:     1, Skipped:     0, Total:     1, Duration: 1 s

Full SeaMetadataE2ETests class — 19/19 pass post-fix.

Files touched

• csharp/src/StatementExecution/StatementExecutionStatement.cs — read canonical names from SHOW KEYS columns; lowercase fallback.
• csharp/test/E2E/StatementExecution/SeaMetadataE2ETests.cs — added SEA_GetPrimaryKeys_ReturnsCanonicalCase_InResultColumns.

Manual verification

• E2E test red → green against pecotesting warehouse
• Full SeaMetadataE2ETests class passes (19/19)
• dotnet build clean on netstandard2.0, net472, net8.0
• pre-commit — environment could not install pip deps (no network from sandbox); pre-existing CI hooks will run on PR