feat: Add ThrottledAt field to HookDelivery

[MarcHuang168]

Adds the throttled_at field to HookDelivery. It is documented in the hook-delivery / hook-delivery-item OpenAPI schemas, and confirmed present in the live list-deliveries and get-delivery responses for a real push event on my fork; it was previously dropped on decode.

Note on verification: the drop was observed end-to-end through ListHookDeliveries on the live API (raw body has the key, decoded struct does not). For GetHookDelivery the live response also carries the key, but its value was null on my fork (the delivery was not throttled), so the typed path can't show a difference there; both methods decode into the same HookDelivery struct, and the non-null case is covered by the updated unit tests.

Updated the existing ListHookDeliveries and GetHookDelivery tests to cover the new field.

Fixes #4517

AI assistance: Claude was used to diff go-github's structs against GitHub's OpenAPI description and to draft the edits; the field was verified against the live API, and the change was applied, tested (script/fmt.sh, script/test.sh, script/lint.sh) and reviewed locally by me.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.53%. Comparing base (1217174) to head (7c8900c).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #4518   +/-   ##
=======================================
  Coverage   98.53%   98.53%           
=======================================
  Files         195      195           
  Lines       17795    17795           
=======================================
  Hits        17534    17534           
  Misses        261      261           

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[gmlewis]

Thank you, @MarcHuang168!
LGTM.
Awaiting second LGTM+Approval from any other contributor to this repo before merging.

cc: @stevehipwell - @alexandear - @Not-Dhananjay-Mishra

[Not-Dhananjay-Mishra]

LGTM

[gmlewis]

Thank you, @Not-Dhananjay-Mishra!
Merging.