Add context manager to Txn and DgraphClientStub.

[shaunpatterson]

Summary

This PR adds comprehensive test coverage for the context manager functionality introduced in PR #185, which adds Python with statement support to the Txn and DgraphClientStub classes.

Background

PR #185 (#185) introduced context manager support but lacked comprehensive tests. The main concern from reviewers was the absence of thorough testing. This PR addresses that concern by adding extensive test coverage.

Tests Added

Txn Context Manager Tests (11 new tests)

1. test_context_manager_auto_commit - Verifies write transactions automatically commit on successful completion
2. test_context_manager_read_only_auto_discard - Verifies read-only transactions automatically discard
3. test_context_manager_exception_handling - Tests exception handling with automatic discard and re-raising
4. test_context_manager_transaction_finished_after_exit - Validates transaction state after context manager exit
5. test_context_manager_multiple_mutations - Tests multiple mutations within a single context manager
6. test_context_manager_query_and_mutate - Tests combined query and mutate operations
7. test_context_manager_invalid_nquad_exception - Tests proper exception handling for invalid operations
8. test_context_manager_read_only_cannot_mutate - Verifies read-only transactions cannot mutate
9. test_context_manager_no_mutations_auto_commit - Tests transactions with no mutations don't error
10. test_context_manager_by_contextlib - Tests context manager via client.begin()
11. test_context_manager_by_class - Tests context manager using Txn class directly

DgraphClientStub Context Manager Tests (8 new tests)

1. test_context_manager_closes_stub - Verifies stub is properly closed after exiting
2. test_context_manager_with_client - Tests using stub context manager with DgraphClient
3. test_context_manager_exception_still_closes - Verifies stub closes even on exceptions
4. test_context_manager_function_wrapper_closes - Tests client_stub() function wrapper
5. test_context_manager_multiple_operations - Tests multiple operations within context manager
6. test_context_manager_nested_with_client_operations - Tests full workflow with nested context managers
7. test_context_manager - Basic context manager usage
8. test_context_manager_code_exception - Exception handling within context manager

Test Coverage

These tests ensure:

• ✅ Proper resource cleanup (commit/discard for transactions, close for stubs)
• ✅ Exception handling with proper cleanup
• ✅ Transaction lifecycle management
• ✅ State validation after context manager exit
• ✅ Integration with existing DgraphClient operations
• ✅ Both read-only and write transaction scenarios
• ✅ Multiple operations within a single context manager
• ✅ Nested context manager scenarios

Related to

• Original PR: Add context manager to Txn and DgraphClientStub. #185
• Addresses reviewer feedback requesting comprehensive tests
• Based on the garylavayou:master branch from the original PR

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