Add coverage for built-in entanglement generation and purification protocols

[SeCuReDmE-main-dev]

Summary

Adds focused tests for built-in entanglement generation and purification protocol registration/selection.

Contributes to #270.

Changes

• Verifies built-in generation protocol registration for Barrett-Kok and Single-Heralded protocols.
• Verifies BBPSSW purification formalism registration and factory selection.
• Keeps this PR test-only; no runtime protocol behavior is changed.

Tests

• py -3.13 -m pytest tests/entanglement_management/test_generation.py tests/entanglement_management/test_purification.py
• py -3.13 -m pytest tests

Intentionally Excluded

• No stabilizer formalism work.
• No theory-comparison timing test from Introduce EG Experiment Theory Comparison to Test Suite #305.
• No runtime refactor.
• No broad documentation rewrite.

Maintainer correction welcome if there is a preferred coverage boundary for #270.