Fix ineffectual assignment in termio test

[Copilot]

The ineffassign linter flagged an unused variable assignment in termio/interruptible_test.go:86.

Changes

• Replace n, err = r.Read(buf) with _, err = r.Read(buf) where the byte count is irrelevant to the test assertion

The test validates only the error type on a subsequent read after cancellation, not the bytes read.

// SUBSEQUENT read should return ErrInterrupted
_, err = r.Read(buf)
if !errors.Is(err, ErrInterrupted) {
    t.Errorf("Expected ErrInterrupted on second read, got %v", err)
}

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