Add abortStream method to Http1StreamManager and ensure streams are released to connection pool

[alextwoods]

Summary

• Make Http1StreamManager robust against external stream cancellation (cancel() + close()) by guaranteeing the connection slot is always released back to the pool
• Add abortStream() API to Http1StreamManager and HttpStreamManager as the correct way for callers to cancel in-flight streams without leaking pool capacity

Problem

When a caller (e.g., the SDK's timeout handler) calls stream.cancel() + stream.close(). (See Java SDK PR #6919 that introduces a new regression) on a stream obtained from Http1StreamManager, the connection slot is permanently leaked from the pool. This happens because Http1StreamManager only releases connections inside its onResponseComplete wrapper, and cancel() + close() can prevent that callback from being delivered — either because the native handle is released before the callback fires, or because the stream was closed before activation made it a no-op.

Under load with aggressive timeouts (e.g., after a network blip), this quickly exhausts the entire pool, causing 100% request failures with no recovery until process restart.

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