fix: terminate unavailable input stream publishers once - #13

When BodyPublishers.ofInputStream receives a supplier that returns null, it subscribes to an empty publisher before reporting IOException. Positive demand can therefore produce both completion and failure; invalid demand can produce two errors, and cancellation during onSubscribe is ignored.

Use a failed subscription with one atomic terminal state so cancellation, invalid demand, and the unavailable-stream error cannot produce conflicting callbacks. Add four public-API regression cases shared with the JVM tests. They record callbacks and check them from the test thread, which catches extra terminal signals that assertions inside worker callbacks can miss.

Validation on macOS arm64 (Scala 3.3.7 / Scala Native 0.5.12 / JVM 25):

• All four new cases pass on Java 25 and fail against unchanged Native source at 42ac91b4.
• With the fix, all 44 publisher checks pass on Native; the same 44 checks pass on JVM 25.
• Cases cover immediate demand, demand after failure, cancellation during subscription, and invalid demand.

This contributes to the JVM-parity work discussed in Requests-Scala #156. Would a US$25 reward be available for this fix and its tests?