refactor!: Rename EditMilestone to UpdateMilestone, split Milestone request bodies into CreateMilestoneRequest and UpdateMilestoneRequest, and pass by value

[JamBalaya56562]

Continues the request-body-by-value work in #3644, this time for the milestone endpoints on IssuesService.

CreateMilestone and EditMilestone reused the 16-field Milestone response type as their request bodies, but per the docs the endpoints accept only four parameters — the other twelve fields (url, number, creator, open_issues, created_at, …) are server-generated. And the two schemas differ:

Field	Create (POST)	Update (PATCH)
title	required	optional
state	optional	optional
description	optional	optional
due_on	optional	optional

So this splits the request body into two dedicated types, following the same approach as #4382, #4401 and the Issues-service splits in #4396/#4400:

• CreateMilestoneRequest — Title is a non-pointer string (no omitempty) since it's required; the rest stay pointers.
• UpdateMilestoneRequest — every field is optional, so all stay pointers.

EditMilestone is renamed to UpdateMilestone to match the docs operation name, Update a milestone (same convention as EditLabel→UpdateLabel in #4400). Both methods now take their body by value, and Milestone is removed from the body-allowed-pointer-types allowlist. The Milestone response type itself is unchanged.

Verified with go build ./..., go vet -tags integration ./test/integration/, gofmt, the full ./github/ test suite (both methods and the eight generated accessors at 100%), and custom-gcl (no paramcheck findings after removing the allowlist entry).

Updates #3644

BREAKING CHANGE: IssuesService.CreateMilestone now takes a new CreateMilestoneRequest (with non-pointer Title) by value, and IssuesService.EditMilestone is renamed to UpdateMilestone and takes a new UpdateMilestoneRequest by value, instead of *Milestone.

cc @jvm986 — this one is in the Issues service, which you'd been working through; since it's been a couple of weeks I picked up the next item, but happy to coordinate again whenever you resume.

[gmlewis]

Thank you, @JamBalaya56562!
LGTM.
Awaiting second LGTM+Approval from any other contributor to this repo before merging.

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

[gmlewis]

Closing and reopening to force all the GitHub Actions workflows to fire properly after today's GitHub outage.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.55%. Comparing base (3f04c1d) to head (65257ec).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #4438   +/-   ##
=======================================
  Coverage   97.55%   97.55%           
=======================================
  Files         194      194           
  Lines       19892    19892           
=======================================
  Hits        19406    19406           
  Misses        268      268           
  Partials      218      218           

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.