feat(csharp): httpPath-based protocol auto-detection (PECO-3055) [SEA]

[eric-wang-1990]

What's Changed

Matches JDBC's compute-type-based protocol selection (DatabricksConnectionContext.getClientTypeFromContext) so the C# driver defaults to SEA for warehouse paths and forces Thrift for GP-cluster paths when adbc.databricks.protocol is not set explicitly. Adds the SEA → Thrift fallback on HTTP 307 from createSession, mirroring JDBC's DatabricksSession.open() handling of DatabricksTemporaryRedirectException.

Resolution order in new DatabricksDatabase.ResolveProtocol:

1. Explicit adbc.databricks.protocol value wins (case-insensitive).
2. Else inspect httpPath:
   • matches .*/o/.+/.+ → "thrift" (GP cluster forced)
   • matches .*/warehouses/.+ or .*/endpoints/.+ → "rest" (SEA default)
3. Else fall back to "thrift" (preserves pre-3055 behavior for callers that omit httpPath, e.g. some integration-test fixtures).

The 307 fallback is gated on auto-detected protocol — explicit "rest" surfaces the 307 to the caller so the SEA-not-available signal is not silently swallowed.

Why

PECO-3055 — M3 explicit deliverable for the SEA-as-default rollout. Without this, the C# driver always defaults to Thrift even on warehouse paths, which is inconsistent with JDBC.

Red → Green proof

Ran against the live pecotesting warehouse (/sql/1.0/warehouses/2f03dd43e35e2aa0).

Before fix (WarehousePath_DefaultsToSeaProtocol_WhenProtocolNotSpecified):

AdbcDrivers.Databricks.Tests.ProtocolAutoDetectionE2ETests.WarehousePath_DefaultsToSeaProtocol_WhenProtocolNotSpecified [FAIL]
  Error Message:
   Assert.IsType() Failure: Value is not the exact type
Expected: typeof(AdbcDrivers.Databricks.StatementExecution.StatementExecutionConnection)
Actual:   typeof(AdbcDrivers.Databricks.DatabricksConnection)

Failed!  - Failed: 1, Passed: 3, Skipped: 0, Total: 4

After fix:

Passed!  - Failed: 0, Passed: 4, Skipped: 0, Total: 4, Duration: 1 s

Full DatabricksConnectionTest regression run (the largest adjacent class):

Passed!  - Failed: 0, Passed: 86, Skipped: 6, Total: 92, Duration: 1 m 1 s

The 6 skips are tests that were previously skipped via TestConfiguration.Protocol == "rest" and now correctly skip via the new DatabricksTestEnvironment.IsResolvedProtocolRest helper (same semantics, just correct for the new default).

Files touched

• csharp/src/DatabricksDatabase.cs — new ResolveProtocol, IsTemporaryRedirectError, OpenThriftConnection, and GetHttpPath helpers; Connect() uses them.
• csharp/test/E2E/ProtocolAutoDetectionE2ETests.cs (new) — four E2E tests covering warehouse-default-SEA, explicit-thrift-override, explicit-rest, GP-cluster-forces-thrift.
• csharp/test/E2E/DatabricksTestEnvironment.cs — adds IsResolvedProtocolRest static helper.
• 21 existing E2E test files — Skip.If(TestConfiguration.Protocol == "rest", ...) predicates replaced with Skip.If(DatabricksTestEnvironment.IsResolvedProtocolRest(TestConfiguration), ...) so they still skip when the resolved (not just explicit) protocol is SEA. Mechanical change, no test semantics altered.

Manual verification

• New ProtocolAutoDetectionE2ETests passes against pecotesting warehouse (4/4)
• DatabricksConnectionTest adjacent regression run passes (86 pass / 6 correctly skipped)
• DriverTests smoke run passes (34 pass / 1 skipped)
• Source builds clean on netstandard2.0 and net8.0 (no warnings, no errors)
• Test project builds clean on net8.0
• pre-commit run — sandbox-network restricted, blocked on installing setuptools from PyPI. Ruff/codespell hooks are Python-only and do not touch the C# files in this PR.

PECO-3055

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