Expand PyBNF tests for BNGsim default and fallback paths

[wshlavacek]

Context

Making BNGsim a default execution backend requires regression coverage for both
the new default path and the legacy/fallback path.

Task

Expand PyBNF tests for BNGsim default and fallback paths.

Acceptance Criteria

• Tests cover BNGsim enabled/default BNGL network execution.
• Tests cover BNGsim enabled/default NF execution.
• Tests cover PYBNF_NO_BNGSIM=1 fallback behavior.
• Tests cover explicit backend selection overriding defaults.
• Tests cover BNGL ode, ssa, and psa where supported.
• Tests cover SBML RoadRunner behavior and BNGsim SBML opt-in without changing
  SBML default behavior prematurely.
• Tests cover Antimony available/unavailable behavior.
• CI can run the relevant subset reliably in clean environments.