[LEP-1814] feat(gpud): make plugin specs update/install more reliable

[gyuho]

• add "gpud run --plugins-init-fail-fast" flag (default false)
• retry on init plugin failures
• update spec comparison logic for empty spec edge cases

By default, GPUd will proceed to join the cluster (or initialize the server)
even if init plugins return unhealthy state (e.g., command exit 1).

Retry on init plugin failures.

[codecov]

Codecov Report

❌ Patch coverage is 26.47059% with 25 lines in your changes missing coverage. Please review.
✅ Project coverage is 66.68%. Comparing base (b787964) to head (13e10d9).

Files with missing lines	Patch %	Lines
pkg/server/server.go	0.00%	11 Missing ⚠️
cmd/gpud/command/command.go	0.00%	4 Missing ⚠️
pkg/session/serve.go	0.00%	3 Missing and 1 partial ⚠️
pkg/session/session.go	42.85%	4 Missing ⚠️
cmd/gpud/run/command.go	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1051      +/-   ##
==========================================
- Coverage   66.71%   66.68%   -0.04%     
==========================================
  Files         314      314              
  Lines       26560    26588      +28     
==========================================
+ Hits        17720    17729       +9     
- Misses       7955     7971      +16     
- Partials      885      888       +3     

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