Read tx context from connection on cursor execute

[vgvoleg]

Fixes #34

Cursors snapshotted connection._tx_context at creation time, so a cursor created before begin() silently ran outside the transaction, and a cursor reused after commit()/rollback() kept executing against a dead transaction.

Cursor._tx_context / AsyncCursor._tx_context are now read-only properties delegating to the connection, matching how psycopg2/asyncpg treat the transaction as connection state. The tx_context constructor argument is dropped — Connection.cursor() no longer passes it.

Tests cover both scenarios from the issue: insert through a pre-begin() cursor is undone by rollback(), and a cursor stays usable after commit().

[Copilot]

Pull request overview

This PR fixes transaction-state drift between Connection and Cursor by making cursors read the active transaction context from the connection at execution time (instead of snapshotting it when the cursor is created). This aligns the driver behavior with common DB-API expectations and addresses the incorrect behavior described in issue #34.

Changes:

• Remove tx_context from Cursor / AsyncCursor constructors and stop passing it from Connection.cursor().
• Implement Cursor._tx_context and AsyncCursor._tx_context as read-only properties that delegate to connection._tx_context.
• Add integration tests covering: (1) cursor created before begin() participates in the later transaction, and (2) cursor remains usable after commit().

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

File	Description
ydb_dbapi/cursors.py	Make cursor transaction context a connection-backed property to avoid snapshotting stale/None tx state.
ydb_dbapi/connections.py	Stop passing tx_context into cursor constructors; rely on connection state instead.
tests/test_cursors.py	Update fake connection stubs to include _tx_context for the new cursor delegation behavior.
tests/test_connections.py	Add regression tests for cursor behavior across begin()/commit()/rollback() ordering.

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