fix: upload stalls with deferred length when size is multiple of chunkSize

[Yanhu007]

Fixes #779

Problem

When using uploadLengthDeferred: true with a Node.js stream whose total size is exactly divisible by chunkSize, the upload hangs forever after the last chunk.

Root cause: After consuming the last aligned chunk, readChunk() is called again. The stream has no more data and emits end, but readChunk only listens for readable events — the promise never resolves.

Timeline

1. slice(0, 7) → reads 7 bytes, done: false
2. slice(7, 14) → reads 7 bytes, stream buffer depleted, done: false
3. slice(14, 21) → buffer empty, readChunk() called → hangs (stream emits end, not readable)

Fix

• Add end event listener in readChunk() that resolves with an empty buffer
• Add readableEnded guard at entry for already-ended streams
• Clean up all three listeners (error, readable, end) on resolution

[Acconut]

Thank you for this report and providing a fix as well. Could you add a unit test covering this case so we'll catch any future regression?