feat(csharp): implement transaction support

[CurtHagenlocher]

What's Changed

Add BigQuery session-based transaction support to the C# ADBC driver, matching the existing Go implementation pattern:

• BigQueryConnection: Add AutoCommit property override that creates/closes BigQuery sessions, and Commit/Rollback methods that issue COMMIT/ROLLBACK TRANSACTION and immediately BEGIN a new transaction
• BigQueryStatement: Pass session_id via ConnectionProperties in QueryOptions when a transaction is active, for both ExecuteQuery and ExecuteUpdate
• Tracing: Add session_id tags and lifecycle events (session_created, session_closed, transaction_started, transaction_committed, transaction_rolled_back) to all transaction operations and session-aware query execution via DiagnosticSource Activity tracing
• BigQueryMockServer: Add session tracking, parse CreateSession and ConnectionProperties from job requests, return SessionInfo in job stats
• TransactionTests: 11 mock server tests covering the full transaction lifecycle (enable/disable autocommit, commit, rollback, error cases, dispose)
• TransactionLiveTests: 5 live integration tests (skipped without credentials) covering commit persistence, rollback discard, multiple commits, and error cases against a real BigQuery instance

[Copilot]

Pull request overview

Adds BigQuery session-based transaction support to the C# ADBC driver (aligning with the Go driver’s approach), including session lifecycle management, session-aware query execution, and corresponding mock/live test coverage.

Changes:

• Implement session-backed AutoCommit, Commit(), and Rollback() behavior in BigQueryConnection using CreateSession, BEGIN/COMMIT/ROLLBACK TRANSACTION, and BQ.ABORT_SESSION.
• Propagate session_id via ConnectionProperties in BigQueryStatement so queries/updates execute within the active transaction session.
• Extend the mock server to parse/return session info and add mock + live test suites to validate the transaction lifecycle.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
csharp/src/AdbcDrivers.BigQuery/BigQueryConnection.cs	Implements session creation/teardown and transaction commands behind AutoCommit/Commit/Rollback.
csharp/src/AdbcDrivers.BigQuery/BigQueryStatement.cs	Attaches session_id connection properties to query/update jobs when a transaction is active.
csharp/test/AdbcDrivers.BigQuery.MockServer/BigQueryMockServer.cs	Adds session parsing/creation and records executed SQL for transaction-related assertions.
csharp/test/AdbcDrivers.BigQuery.Tests/MockServer/TransactionTests.cs	Adds mock-server tests covering autocommit toggling and commit/rollback behavior.
csharp/test/AdbcDrivers.BigQuery.Tests/TransactionLiveTests.cs	Adds live integration tests validating commit persistence and rollback discard semantics.

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