[0%] Phase 6: Wire CritterMapper into MorphiaDatastore

[evanchooly]

Parent: #4179
Blocked by: #4187

Goal

Complete the config-driven mapper selection in MorphiaDatastore.

Details

See Phase 6 in planning doc

Tasks

• Update MorphiaDatastore.createMapper() factory to dispatch on config.mapper()
• Verify polymorphic copy() works in session datastore pattern
• Verify importModels() (ServiceLoader/EntityModelImporter) works with both mapper types
• Run existing test suite with LEGACY (must be green)
• Run focused tests with CRITTER

Progress Tracking

phase_percentage = (completed_tasks / 5) * 100, rounded to nearest integer
gh issue edit 4188 --repo MorphiaOrg/morphia --title "[XX%] Phase 6: Wire CritterMapper into MorphiaDatastore"

parent_percentage = (total_completed_tasks_across_all_phases / 55) * 100, rounded to nearest integer
gh issue edit 4179 --repo MorphiaOrg/morphia --title "[XX%] Integrate critter-core into morphia-core"

When all tasks are complete (100%), close this issue and check its box in #4179.