Increase optimizer submodule test coverage

[Copilot]

Summary

Increase coverage for low-tested optimizer submodules by adding a focused, deterministic optimizer test suite.

Changes

• Added tests/test_optimizer_submodules.py.
• Covered key behavior in:
  • py3plex.optimizer.plan_nodes
  • py3plex.optimizer.logical_plan
  • py3plex.optimizer.cost_model
  • py3plex.optimizer.physical_plan
  • py3plex.optimizer.rules
  • py3plex.optimizer.optimizer
• Added tests for:
  • logical plan helper extraction and select-plan chain construction
  • network stats extraction and fallback behavior
  • cost estimate arithmetic and operator-level cost estimation
  • physical operators (layer pushdown, top-k, early limit) and physical plan serialization
  • rewrite rule behavior and rule engine traversal
  • optimizer metadata and optimize_query one-shot flow

Coverage

Measured with:

• PYTEST_DISABLE_PLUGIN_AUTOLOAD=1 coverage run -m pytest tests/test_optimizer_submodules.py -q
• coverage report -m py3plex/optimizer/*.py

Results for py3plex/optimizer:

• Baseline (before): 0% total (with non-optimizer test target)
• After: 81% total

Per-file after:

• __init__.py: 100%
• plan_nodes.py: 100%
• logical_plan.py: 94%
• cost_model.py: 89%
• optimizer.py: 93%
• physical_plan.py: 77%
• rules.py: 66%

Validation

• python -m ruff check tests/test_optimizer_submodules.py
• PYTEST_DISABLE_PLUGIN_AUTOLOAD=1 python -m pytest tests/test_optimizer_submodules.py -q
• parallel_validation run (Code Review + CodeQL) after updates