feat(csharp): add preview endpoint support for SEA

[jadewang-db]

Summary

• Add UsePreviewEndpoint (adbc.databricks.rest.use_preview_endpoint) configuration parameter to route SEA requests to /2.0/preview/sql/... paths instead of the default /api/2.0/sql/... paths
• Wire the parameter through StatementExecutionConnection into StatementExecutionClient, which selects the appropriate endpoint constants for sessions and statements
• Add unit tests verifying endpoint routing for all client operations (create/delete session, execute/get/cancel/close statement, get result chunk) and connection-level property integration

Test plan

• Unit tests for StatementExecutionClient constructor with usePreviewEndpoint true/false
• Unit tests verifying all 7 client operations use preview paths when enabled
• Unit tests verifying standard paths are used when preview is disabled
• Connection-level tests verifying property parsing and default behavior

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