GUACAMOLE-827: Fix BlobWriter race condition in chunk upload.

[GokulRG]

Fixes a critical race condition where multiple blob chunks could be read and sent simultaneously without proper ACK synchronization.

Changes:

• Add in-flight tracking to prevent concurrent chunk operations
• Register ACK handler BEFORE sendData() to eliminate race window
• Add duplicate ACK prevention with ackHandled flag
• Use setTimeout for non-recursive chunk scheduling
• Reset state on FileReader errors

[GokulRG]

This MR should address this longstanding bug where subsequent chunks are sent to the BlobWriter before the previous chunk is fully processed resulting in incomplete file transfers with missing chunks.

[necouchman]

@GokulRG Thanks for submitting this. Please fix the style of the PR title and the commit message by adding a ":" after GUACAMOLE-827.

[GokulRG]

@GokulRG Thanks for submitting this. Please fix the style of the PR title and the commit message by adding a ":" after GUACAMOLE-827.

Of course. Thank you!

[GokulRG]

Hi @necouchman, Were you able to look at this MR?

[eugen-keeper]

I believe the fix is correct and necessary. LGTM.
@GokulRG please remove the space before the colon in the PR title and add periods at the end of the commit and PR titles.

[necouchman]

@GokulRG Seems like this should be re-based against either patch or staging/1.6.1?

[GokulRG]

@eugen-keeper Done. Thanks a lot for the approval.

[GokulRG]

@necouchman Please let me know if it looks good now. Rebased and changed the base branch to staging/1.6.1