Address Nexus Operation command issues

[Quinn-With-Two-Ns]

What changed?

Address some issues found in the Nexus Operation command like:

• Operation ID should be required
• Some missing fields on start

Checklist

Stability

• Breaking changes are marked with 💥 in the PR title and release notes
• Changes to JSON output (-o json / -o jsonl) are treated as breaking changes

Design

• This feature does not depend on Cloud-only APIs or behavior (it works against an OSS server)
• New commands follow temporal <noun> <verb> structure (e.g. temporal workflow start)
• New flags are named after the API concept, not the implementation mechanism (good: --search-attribute, bad: --index-field)
• New flags don't duplicate an existing flag that serves the same purpose
• New flags do not have short aliases without strong justification
• Experimental features are marked with (Experimental) in commands.yaml

Help text (see style guide at the top of commands.yaml)

• All flags shown in help text and examples are implemented and functional
• Summaries use sentence case and have no trailing period
• Long descriptions end with a period and include at least one example invocation
• Examples use long flags (--namespace, not -n), one flag per line
• Placeholder values use YourXxx form (YourWorkflowId, YourNamespace)

Behavior

• Results go to stdout; errors and warnings go to stderr
• Error messages are lowercase with no trailing punctuation

Tests

• Added functional test(s) (SharedServerSuite)
• Added unit test(s) (func TestXxx) where applicable

Manual tests

Setup

  temporal operator nexus endpoint create                                                                                                  
      --name my-endpoint                                                                                                                 
      --target-namespace default
      --target-task-queue my-tq

Happy path

  $ temporal nexus operation start                                                                                                         
      --endpoint my-endpoint      
      --service my-service                                                                                                                 
      --operation my-op                                                                                                                  
      --operation-id my-op-1
      --input '"hello"'     
  Started Nexus Operation:                                                                                                                 
    Endpoint    my-endpoint
    Service     my-service                                                                                                                 
    Operation   my-op                                                                                                                    
    OperationId my-op-1
    RunId       7e7c8b2a-...-...
    Namespace   default

Error case

  $ temporal nexus operation start                                                                                                         
      --service my-service                                                                                                                 
      --operation my-op   
      --operation-id my-op-2                                                                                                               
  Error: required flag(s) "endpoint" not set                                                                                             

Composition — start an operation, then fetch its result, then list/describe:

  $ temporal nexus operation start
      --endpoint my-endpoint                                                                                                               
      --service my-service                                                                                                               
      --operation my-op                                                                                                                    
      --operation-id my-op-3
      --input '"world"'                                                                                                                    
                                                                                                                                         
  $ temporal nexus operation result --operation-id my-op-3
  Results:                                                                                                                                 
    Status  COMPLETED
    Result  "got: world"                                                                                                                   
                                                                                                                                         
  $ temporal nexus operation describe --operation-id my-op-3
    OperationId            my-op-3
    RunId                  7e7c8b2a-...-...                                                                                                
    Endpoint               my-endpoint
    Service                my-service                                                                                                      
    Operation              my-op                                                                                                         
    Status                 Succeeded                                                                                                       
    ...