test(contrib/drivers/gaussdb): add transaction, where, hook, ctx test coverage

[lingcoder]

Summary

• Port 4 test files from PgSQL driver to GaussDB driver to align test coverage
• Add gaussdb_z_unit_transaction_test.go (40 tests): nested transactions, savepoints, rollback, panic recovery, context propagation
• Add gaussdb_z_unit_model_where_test.go (35 tests): comprehensive Where clause combinations (map, slice, struct, pointer, operators, nil, empty)
• Add gaussdb_z_unit_feature_hook_test.go (6 tests): model hook callbacks (Select/Insert/Update/Delete)
• Add gaussdb_z_unit_feature_ctx_test.go (5 tests): context propagation, timeout, logging with context
• Remove old Test_Model_Where (2 sub-tests) from gaussdb_z_unit_model_test.go, replaced by comprehensive version in dedicated where test file (35 tests)
• 86 new test functions, ~3,224 net new lines

Test plan

• go build ./... passes
• gofmt and gci applied
• No remaining pgsql references in new files
• Run go test -v -run "Test_TX_" -count=1 against GaussDB instance
• Run go test -v -run "Test_Model_Where" -count=1 against GaussDB instance
• Run go test -v -run "Test_Model_Hook" -count=1 against GaussDB instance
• Run go test -v -run "Test_Ctx" -count=1 against GaussDB instance

ref #4689

[Copilot]

Pull request overview

This PR expands the GaussDB driver test suite by porting and extending unit/feature tests (transactions, WHERE clause handling, hooks, and context propagation) to better align coverage with other SQL drivers.

Changes:

• Added extensive transaction tests covering nested transactions, savepoints, propagation modes, panic recovery, and isolation levels.
• Added a dedicated and comprehensive Model.Where/WherePri/... test file, replacing a smaller Test_Model_Where in the existing model test file.
• Added feature tests for model hooks and context propagation behaviors.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 6 comments.

Show a summary per file

File	Description
contrib/drivers/gaussdb/gaussdb_z_unit_transaction_test.go	Adds broad transaction behavior coverage (nested TX, savepoints, propagation, isolation).
contrib/drivers/gaussdb/gaussdb_z_unit_model_where_test.go	Adds comprehensive WHERE/WHEREPRI coverage across many input forms and operators.
contrib/drivers/gaussdb/gaussdb_z_unit_model_test.go	Removes the older minimal Test_Model_Where now covered by the new dedicated file.
contrib/drivers/gaussdb/gaussdb_z_unit_feature_hook_test.go	Adds hook handler tests for Select/Insert/Update/Delete behaviors.
contrib/drivers/gaussdb/gaussdb_z_unit_feature_ctx_test.go	Adds tests around context propagation, logging keys, transactions, and timeouts.

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[lingcoder]

@gqcn 感谢review，这里解释一下几个设计选择：

关于事务测试忽略Insert错误：
测试重点在PropagationRequiresNew的事务隔离性 - 验证tx2的独立事务不受外层tx.Rollback()影响。MySQL驱动在同样场景下也是忽略Insert返回值直接验证事务行为，为了保持Layer 2测试跨驱动的可移植性，GaussDB这里沿用了相同模式。

关于Hook测试用db.Model：
Hook测试要验证的是Hook替换机制本身，用db.Model可以避免Hook嵌套干扰测试结果。Context传递的测试在Test_Model_Ctx里专门覆盖，不用在Hook测试里重复。而且Hook函数通过in参数已经拿到完整上下文了，这个设计跟MySQL的Hook测试保持一致。

关于Ctx测试忽略返回值：
这个测试验证的是Context传播链路（traceId能不能正确传到日志里），不关心查询本身的结果。断言只检查日志内容，所以查询成不成功不影响验证逻辑。MySQL的Ctx测试也是同样设计。

这些模式都是为了跟MySQL基线对齐，方便后续横向比对各驱动的测试覆盖率。👍