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. The value-receiver marshaler supports both values and pointers without mutation. Creates and responses are unchanged.

The Go 1.26.0 encoding/json repro shows that *string behaves identically with omitempty and omitzero: nil omits the field, new("network-id") sends a string, and new("") sends "". Neither tag produces JSON null. Empty-string behavior is preserved for compatibility, but empty-string detachment is unverified. Both PATCH schemas allow null: organization and enterprise.

Regression tests cover omission, assignment, empty-string compatibility, explicit null, flag precedence, preservation of other fields, value/pointer marshaling, request immutability, and both native PATCH bodies. The null cases fail without the marshalers. On Go 1.26.0, runner-group race tests, all-module script/test.sh race tests, script/fmt.sh, script/generate.sh, script/lint.sh, and OpenAPI metadata validation pass. No live API writes were made for this revision.

This revises the earlier omitzero implementation and still needs maintainer agreement on explicit-null removal. Needed by integrations/terraform-provider-github#3274, which remains gated on a merged and released SDK fix.

Copilot assisted with implementation, tests, and this description.

[google-cla]

Thanks for your pull request! It looks like this may be your first contribution to a Google open source project. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

View this failed invocation of the CLA check for more information.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.59%. Comparing base (c5074ea) to head (6cf8993).
⚠️ Report is 20 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4541      +/-   ##
==========================================
+ Coverage   98.54%   98.59%   +0.04%     
==========================================
  Files         196      197       +1     
  Lines       17938    18344     +406     
==========================================
+ Hits        17677    18086     +409     
+ Misses        261      258       -3     

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[gmlewis]

@austenstone - just like in another recent PR, we now have a CLA problem because an agent signed a commit that was pushed to this PR.

The following contributors were found for this pull request:

✅ https://github.com/google/go-github/commit/b0fc528fc916672eb5d41e81defdbb91c1aa4416 Author: @austenstone <aus******ne​@github.com>
❌ https://github.com/google/go-github/commit/b0fc528fc916672eb5d41e81defdbb91c1aa4416 Co-Author: @Copilot <223556219+Copilot​@users.noreply.github.com>

So now, you can either clean up the commits locall and force-push the changes to this PR to get rid of the bot's commits or you can close this PR and open a brand new one. Your choice, but we cannot continue this PR with the current CLA state, unfortunately.

[gmlewis]

Thank you, @austenstone.
One wording suggestion, otherwise, LGTM.

cc: @stevehipwell - @Not-Dhananjay-Mishra

[gmlewis]

How about this?

Suggested change

	For optional primitive fields where the zero value has API semantics, use a
	pointer with `omitempty`. A nil pointer omits the field, while a pointer to the
	zero value includes it, such as `false`, `0`, or `""`.
	Neither `omitempty` nor `omitzero` makes a pointer to a zero value encode as
	JSON `null`. When an update must distinguish omission, assignment, and explicit
	removal using `null`, follow `UpdateTeamRequest.RemoveParentTeam`: add a Go-only
	removal flag and a value-receiver `MarshalJSON` method. This supports marshaling
	both request values and pointers without changing the request.
	Optional pointer fields should use `omitempty`: a nil pointer omits the field,
	while a pointer to a zero value such as `false`, `0`, or `""` includes it.
	`omitzero` behaves identically for pointers, so prefer `omitempty`.
	Neither tag can send JSON `null`, so an update that must distinguish omission,
	assignment, and removal follows `UpdateTeamRequest.RemoveParentTeam` in
	`github/teams.go`: a Go-only removal flag tagged `json:"-"`, plus a
	value-receiver `MarshalJSON` that also serves `*T` call sites. `new("")` sends
	an empty string, not `null`.