KB notifications endpoint improvements

[lferran]

Description

• Improve notification models to be more precise and explicit (were too generic before)
• Control when the stream is closed with a timeout
• Extend the payload to indicate whether there were processing errors
• Add documentation
• Return an explicit 204 if the endpoint is called on standalone mode

How was this PR tested?

Unit and integration tests

[lferran]

related tickets [sc-8413] [sc-8414]

[shortcut-integration]

This pull request has been linked to:

• Shortcut Story #8413: proper error status on notification endpoint timeout
• Shortcut Story #8414: Error property needed on processing notifications

[codecov]

Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (8e42cb2) 82.18% compared to head (b799936) 82.19%.

Files	Patch %	Lines
nucliadb/nucliadb/reader/reader/notifications.py	94.73%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1735      +/-   ##
==========================================
+ Coverage   82.18%   82.19%   +0.01%     
==========================================
  Files         334      334              
  Lines       19570    19587      +17     
==========================================
+ Hits        16084    16100      +16     
- Misses       3486     3487       +1     

Flag	Coverage Δ
ingest	69.05% <ø> (ø)
nucliadb	70.32% <19.04%> (-0.07%)	⬇️
reader	79.41% <95.23%> (+0.39%)	⬆️
sdk	40.58% <ø> (ø)
search	79.04% <ø> (ø)
standalone	88.29% <ø> (ø)
train	63.71% <ø> (ø)
utils	81.25% <ø> (ø)
writer	85.15% <ø> (ø)

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