fix: Allow removing runner group networks

[austenstone]

Add RemoveNetworkConfiguration to organization and enterprise runner-group update requests, following UpdateTeamRequest.RemoveParentTeam. Setting it sends "network_configuration_id": null and takes precedence over a supplied ID. Existing omission and string behavior is unchanged. Creates and responses are untouched.

Both PATCH schemas allow null: organization and enterprise.

Needed by integrations/terraform-provider-github#3274.

Regression tests cover omission, strings, explicit null, flag precedence, preservation of other fields, value/pointer marshaling without mutation, and both native PATCH methods. Targeted runner-group tests with -race, formatting, lint, and generator checks pass on Go 1.26.0. No live detachment was tested.

Copilot assisted with implementation, tests, and this description.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.54%. Comparing base (c5074ea) to head (c4d4c88).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #4541   +/-   ##
=======================================
  Coverage   98.54%   98.54%           
=======================================
  Files         196      196           
  Lines       17938    17956   +18     
=======================================
+ Hits        17677    17695   +18     
  Misses        261      261           

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[gmlewis]

If I'm not mistaken, we can solve this in a different way without resorting to adding a new "hidden" field to the struct...

We simply change line 62 above to this:

NetworkConfigurationID   *string  `json:"network_configuration_id,omitzero"`

and then when the user doesn't initialize the field (it is nil), the field will be ignored.
However, if the user wants to REMOVE the network configuration, they would send new("").

I think we should put this in CONTRIBUTING.md on line 418 right after this comment:

For optional boolean fields where you need to distinguish between `false`
and "not set", use `*bool` with `omitzero`.