feat: add application-level keepalive to prevent ALB idle connection timeouts

[JoshVanL]

AWS ALBs do not forward HTTP/2 PING frames, causing idle gRPC connections to be closed. This adds a background loop that periodically calls the existing Hello RPC as application-level traffic to keep the connection alive through L7 load balancers.

[Copilot]

Pull request overview

Adds an application-level gRPC keepalive loop in the Durable Task worker to reduce the chance of idle HTTP/2 connections being closed (e.g., by AWS ALBs) while the worker is waiting on the long-lived getWorkItems stream.

Changes:

• Introduces a scheduled keepalive task that periodically calls a unary RPC (hello) on the sidecar.
• Starts the keepalive loop after opening the work-item stream and stops it on disconnect/retry and on close().
• Adds supporting constants/imports and scheduler lifecycle management.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 79.51%. Comparing base (f63a4a4) to head (44b4e4c).

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #1695      +/-   ##
============================================
- Coverage     79.53%   79.51%   -0.02%     
- Complexity     2193     2194       +1     
============================================
  Files           237      237              
  Lines          6577     6577              
  Branches        730      730              
============================================
- Hits           5231     5230       -1     
- Misses          990      992       +2     
+ Partials        356      355       -1     

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[javier-aliaga]

ey @JoshVanL why is this inside the durable worker? I think it does not belong there but on the class we build the channel so all component will benefit from it not just the worflow