Add support for module tests

[xsedla1o]

Adds lightweight testing utilities for DP3 secondary modules so app developers can unit-test module hook behavior without running a full DP3 instance.

Secondary modules were difficult to test because they normally depend on DP3 runtime wiring. This branch provides a unittest-friendly harness that loads real app entity config, captures hook registrations, and lets tests invoke hooks directly with validated DP3 task/datapoint objects.

Examples

To showcase potential usefulness of such helpers, usable test suites were drafted for select internal apps:

• ADiCT
• Amfora (tests pass when this branch of DP3 is installed, CI fails as installs from main)
• NERD2

Changes

• Added dp3.testing helpers, centered around DP3ModuleTestCase.
• Added fake/capturing callback registrar for module hook registration tests.
• Added helpers for building tasks/datapoints, running hooks, scheduler callbacks, updater hooks, and snapshot/correlation
  hooks.
• Added assertion helpers for emitted tasks/datapoints and mutated snapshot records.
• Added timeseries datapoint test helper.
• Documented module testing workflow in docs.
• Added tests covering the new utilities.
• Added validation preventing empty DataPointTasks that would do no work, while preserving internal synthetic-task use cases.