feat: Add image_gen to HostedRunner and UpdateHostedRunnerRequest

[breken-ai]

Fixes #4532

What

The GitHub-hosted runner API supports image_gen on create, update, and read (it was added for create in #3973), but go-github only models it on CreateHostedRunnerRequest. The field can be set at create time but never read back or patched: HostedRunner (the response type for GetHostedRunner, CreateHostedRunner, UpdateHostedRunner, DeleteHostedRunner, ListHostedRunners, and the EnterpriseService equivalents) and UpdateHostedRunnerRequest both lack it. As the issue notes, this breaks terraform-provider-github round-trips: an import records false, and an apply with true destroys and recreates the pool.

Verified against current master (2433615): github/actions_hosted_runners.go has ImageGen only on CreateHostedRunnerRequest (:103); HostedRunner (:30) and UpdateHostedRunnerRequest (:107) do not.

Change

• HostedRunner: add ImageGen *bool \json:"image_gen,omitempty"``
• UpdateHostedRunnerRequest: add ImageGen *bool \json:"image_gen,omitempty"``
• script/generate.sh run per CONTRIBUTING - github-accessors.go and github-accessors_test.go regenerated (accessor + accessor test for both new fields; no hand-written surface beyond the two field lines)
• New round-trip marshal tests in the repo's standard testJSONMarshal style: TestHostedRunner_Marshal and TestUpdateHostedRunnerRequest_Marshal, each covering the empty struct and a fully populated struct including image_gen

Testing

• TestHostedRunner_Marshal, TestUpdateHostedRunnerRequest_Marshal: PASS
• Existing hosted-runner tests re-run against the change (TestActionsService_ListHostedRunners, TestActionsService_GetHostedRunner, TestActionsService_UpdateHostedRunner, TestActionsService_ListHostedRunnersIter): PASS
• gofmt -l github/: clean
• Disclosure: the full github package test binary OOMs at compile time in this memory-constrained sandbox (2 GB), so the complete suite was not run locally; the targeted tests above all pass and CI will cover the rest.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.54%. Comparing base (2433615) to head (9d5cfe1).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #4544   +/-   ##
=======================================
  Coverage   98.54%   98.54%           
=======================================
  Files         196      196           
  Lines       17938    17938           
=======================================
  Hits        17677    17677           
  Misses        261      261           

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[gmlewis]

@breken-ai - please solve the issue reported here: https://github.com/google/go-github/pull/4544/checks?check_run_id=103155993223
and then we can proceed with this PR.