Expand Database module integration coverage across DB and ORM contracts

[Copilot]

This PR extends goravel/example database integration tests to cover missing contract-level behaviors in both query builder (facades.DB()) and ORM (facades.Orm()), beyond the existing basic CRUD and restore cases. The goal is broader behavioral coverage in tests/feature/db_test.go and tests/feature/orm_test.go so framework regressions surface via failing tests.

• DB contract coverage (tests/feature/db_test.go)

  • Added transaction semantics coverage:
    • rollback path does not persist inserted rows
    • commit path persists inserted rows
  • Added query helper coverage:
    • Count, Exists, DoesntExist, WhereNotNull(...).Count()
    • multi-row insert + RowsAffected assertions
  • Added error-path coverage:
    • FirstOr callback execution only on miss
    • callback error propagation
    • FindOrFail failure on missing row

• ORM contract coverage (tests/feature/orm_test.go)

  • Added FirstOrCreate flow:
    • create-on-miss
    • find-on-hit without duplicate row creation
  • Added UpdateOrCreate flow:
    • create-on-miss
    • update-on-hit with stable identity and single-row invariant

• Cross-driver behavior

  • New test cases are implemented in existing suites so they run under the repository’s multi-driver database test harness (postgres/mysql/sqlserver), consistent with current feature test structure.

func (s *DBTestSuite) TestTransaction() {
	tx, _ := facades.DB().BeginTransaction()
	_, _ = tx.Table("users").Insert(map[string]any{"name": "rollback"})
	_ = tx.Rollback()

	exists, _ := facades.DB().Table("users").Where("name", "rollback").Exists()
	s.False(exists) // rollback should not persist data
}

Original prompt

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This section details on the original issue you should resolve

<issue_title>Add full tests for the Database module</issue_title>
<issue_description>### Before feedback (在反馈之前)

• There are no features that I want to submit in Issues (当前 Issues 中没有我将要提交的新功能)

Describe Feature (描述功能)

Before feedback (在反馈之前)

• There are no features that I want to submit in Issues (当前 Issues 中没有我将要提交的新功能)

Describe Feature (描述功能)

We want to add integration tests for goravel/framework's features to ensure they work as expected.

1. Add full tests for the Database module in goravel/example based on the interface: https://github.com/goravel/framework/tree/master/contracts/database.
2. The tests can be added in https://github.com/goravel/example/tree/master/tests/feature/db_test.go and orm_test.go. There are some existing cases, try to find missing flows. Feel free to add a controller in https://github.com/goravel/example/tree/master/app/http/controllers if a request is required.
3. Let tests fail if there are bugs in the module.
4. Create a branch for the PR based on the master branch, and merge the PR to the master branch.</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Add full tests for the Database module goravel#930

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