Fix CSA_motor_efficiency UnboundLocalError for unsupported pole counts

[yuhuavit-hash]

CSA_motor_efficiency raises an UnboundLocalError (instead of a clear error) when the requested pole count has no standard table: 8-pole motors have no high-efficiency values, and pole counts other than 2/4/6/8 are not in any table.

>>> from fluids.pump import CSA_motor_efficiency
>>> from fluids.constants import hp
>>> CSA_motor_efficiency(100*hp, poles=8, high_efficiency=True)
UnboundLocalError: cannot access local variable 'efficiency'

This adds input validation up front, raising a ValueError with a clear message for unsupported pole counts (matching the style of current_ideal's phase validation), and documents the supported pole counts in the docstring.

Changes

• fluids/pump.py: validate poles before table lookup
• tests/test_pump.py: add boundary cases (poles=8 + high_efficiency, poles=3)

Verification

• python -m pytest tests/test_pump.py — 12 passed
• python -m pytest -m "not slow" — 1764 passed