Add pandas 2+3 CI matrix testing

[MaxGhenis]

Summary

• Adds pandas-version: ["2", "3"] to the CI test matrix in the Test job
• After installing dependencies, forces the specific pandas major version so tests run against both pandas 2.x and 3.x
• Ensures policyengine-core remains compatible with both pandas 2.x and 3.x
• Follows up on the recent pandas 3.0 compatibility fixes (Add pandas 3.0 compatibility fixes #424, Fix pandas 3.0 KeyError in Enum.encode() for Series with non-integer index #428, Fix pandas 3.0 StringArray compatibility in ParameterNodeAtInstant #430)

Test plan

• CI passes with pandas 2 matrix (ubuntu + windows, py3.12 + py3.13)
• CI passes with pandas 3 matrix (ubuntu + windows, py3.12 + py3.13)

🤖 Generated with Claude Code

[MaxGhenis]

Closing — draft from Feb 3 with conflicts. The pandas 2+3 CI matrix would double CI jobs for marginal benefit, and the workflow has changed significantly since (uv migration, Python 3.14 added, changelog system change). Can revisit with a fresh PR if needed.