fix: send explicit eot from stream receiver task to stream execute task

[vaibhavtiwari33]

What this PR does / why we need it

Whenever the stream receiver task, a single task responsible for receiving responses from udf server and streaming them back to the respective execute tasks, drops the sender used for streaming back responses to execute tasks; the execute tasks see that as a signal that all the responses have been sent by the receiver task.

We should send an explicit EOT from from the receiver task to let the execute task know that we're done sending responses.

Related issues

Fixes #3417

Testing

• Updated the unit tests
• Testing using the validation platform

Special notes for reviewers

Anything notable for review (risk, rollout, follow-ups).

[codecov]

Codecov Report

❌ Patch coverage is 85.71429% with 12 lines in your changes missing coverage. Please review.
✅ Project coverage is 82.68%. Comparing base (e07240b) to head (df455a0).

Files with missing lines	Patch %	Lines
rust/numaflow-core/src/mapper/map/stream.rs	85.71%	12 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3418      +/-   ##
==========================================
+ Coverage   82.64%   82.68%   +0.03%     
==========================================
  Files         307      307              
  Lines       76051    76089      +38     
==========================================
+ Hits        62853    62911      +58     
+ Misses      12645    12620      -25     
- Partials      553      558       +5     

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