Add optional heuristic weight and tie-break support to A*

[christiefhyang]

Reference issue

None.

What does this implement/fix?

This PR adds optional Weighted A* settings to AStarPlanner while preserving the default behavior.

Changes:

• Add heuristic_weight, defaulting to 1.0.
• Add tie_breaker, defaulting to None.
• Support tie_breaker="larger_g" to prefer nodes farther from the start when priorities are equal.
• Add last_expanded_node_count for benchmarking without changing the planning() return values.
• Add tests for default compatibility, weighted planning, and invalid options.

Additional information

The default constructor path keeps the same behavior as before. In local benchmarking, weighted variants reduced expanded nodes on the default demo scenario while preserving the path length in that run.

CheckList

• Did you add a unittest for your new example or defect fix?
• Did you add documents for your new example?
• All CIs are green? (You can check it after submitting)