Remove deprecated source simulation helpers and migrate tests to generator/injector fixtures

[Copilot]

The deprecated source-generation helpers in sotrplib/sims/sources/core.py were still coupling source creation and injection by returning both injected maps and generated sources together. This change removes that legacy path and updates the tests to use the current split model: generate sources first, then inject them.

• Remove deprecated simulation API

  • Delete the legacy source simulation classes from sotrplib/sims/sources/core.py
  • Keep ProcessableMapWithSimulatedSources, which is still used by the injector path

• Migrate tests to the current simulation flow

  • Replace test usage of RandomSourceSimulation* helpers with shared fixtures built from:
    • FixedSourceGenerator
    • PhotutilsSourceInjector
  • Move the shared setup into tests/conftest.py to avoid duplicating the same generator/injector wiring across suites

• Stabilize injected-map fixtures

  • Build test sources inside a buffered bounding box rather than directly on the map edges
  • Preserve existing forced-photometry and pointing expectations that assume thumbnails remain fully in-bounds

• Affected test suites

  • tests/test_simulation_pipeline
  • tests/test_forced_photometry
  • tests/test_pointing

Example of the new test setup pattern:

generator = FixedSourceGenerator(
    min_flux=min_flux,
    max_flux=max_flux,
    number=n_sources,
    catalog_fraction=1.0,
)
simulated_sources, catalog = generator.generate(box=buffered_box)

injector = PhotutilsSourceInjector(
    fwhm_uncertainty_fraction=fwhm_uncertainty_fraction
)
_, new_map = injector.inject(
    input_map=input_map,
    simulated_sources=simulated_sources,
)