refactor(planner): split monolithic planner_test.go into focused test…

[bella-247]

This pull request adds comprehensive unit and integration tests for the planner package, focusing on the correctness and robustness of the BuildPlan function for database table generation order, especially in the presence of foreign key cycles, nullable constraints, and real-world schemas.

Cycle handling and edge cases:

• Adds tests to cycles_test.go that verify BuildPlan correctly handles various cycle scenarios, including cycles with nullable and non-nullable foreign keys, self-referencing tables, composite foreign keys, and mixed acyclic/cyclic graphs. These tests ensure that cycles with nullable breakpoints are deferred properly, and errors are raised when cycles are unresolvable.

Integration and realistic schema coverage:

• Introduces integration tests in integration_test.go that check row count propagation, deferred foreign key metadata, and the ability to generate a valid plan for a realistic ecommerce schema with multiple tables and dependencies.
• Verifies that all tables are present in the plan, dependency order is respected, and deferred columns are handled for self-referencing and cyclic cases in the ecommerce schema.

Test coverage and helper utilities:

• Provides helper functions for extracting table names from the plan and constructing realistic schema models, improving test readability and maintainability.

These additions significantly increase test coverage for complex schema scenarios and help ensure the correctness of the table generation planning logic.… files

Extract test suites into dedicated files:

• cycles_test.go — Tarjan's SCC and cycle resolution tests
• integration_test.go — full pipeline integration tests
• planner_test.go now contains only core planner tests

What does this PR do?

Why is it needed?

Closes #

How was it tested?

Does it change any existing behavior?

Checklist

• Tests added or updated
• make test passes (go test ./... -race)
• make lint passes (golangci-lint run ./...)
• No cross-stage imports (parser doesn't import generator, etc.)
• No time.Now() or unseeded randomness in the generation pipeline
• Error messages follow the format in SRS §12.2
• Exported functions have doc comments
• Golden files updated if output format changed (make test-golden UPDATE=true)

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