test: add regression guard for nil-error invariant on failure conditions

[Copilot]

The nil-check guard (err != nil && c.options.isFailure(err)) in hoglet.go establishes a contract that custom failure conditions are never invoked with nil errors, but nothing enforced this at the test level.

Changes

• Regression test (TestCircuit_failure_condition_never_called_with_nil): registers a failure condition that panics on nil, wraps a function returning a nil error, and asserts no panic — any removal of the nil guard will cause an immediate, obvious failure.

panicOnNil := func(err error) bool {
    if err == nil {
        panic("failure condition must never be called with nil error")
    }
    return true
}

b, err := NewCircuit(nil, WithFailureCondition(panicOnNil))
require.NoError(t, err)

assert.NotPanics(t, func() {
    _, _ = Wrap(b, noop)(t.Context(), noopInSuccess)
})

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