refactor!: Split Key request bodies into CreateDeployKeyRequest, CreateUserKeyRequest and CreateSSHSigningKeyRequest and pass by value

[JamBalaya56562]

Continues the request-body-by-value work in #3644, this time for the three key-creation endpoints that shared the Key response type as their request body.

Per the docs, each endpoint accepts only a small request schema — the other six Key fields (id, url, verified, created_at, added_by, last_used) are server-generated — and the schemas differ per resource:

Method	Endpoint	key	title	read_only
RepositoriesService.CreateKey	Create a deploy key	required	optional	optional
UsersService.CreateKey	Create a public SSH key	required	optional	—
UsersService.CreateSSHSigningKey	Create a SSH signing key	required	optional	—

So this splits the body into three per-resource request types — CreateDeployKeyRequest (with ReadOnly), CreateUserKeyRequest and CreateSSHSigningKeyRequest — following the same approach as #4406/#4425/#4432/#4434/#4444. Key is a non-pointer string in all three since it's required everywhere. The methods keep their names (the Create verb already matches the docs, and renaming only the create methods would break consistency with the surrounding Get/List/DeleteKey families).

Also fixed in passing, since the signature change touches the exact lines: the failure case in TestUsersService_CreateSSHSigningKey had a pre-existing copy-paste bug — it called Users.CreateKey (a different method) with methodName = "CreateKey"; it now exercises CreateSSHSigningKey as intended.

Notes:

• The Key response type is unchanged — it stays the response for ListKeys/GetKey/both create methods, so its fields keep pointer semantics. SSHSigningKey is untouched.
• test/integration/users_test.go builds under the root module with -tags integration, so its Users.CreateKey call is updated too (verified with go vet -tags integration).

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

Updates #3644

BREAKING CHANGE: RepositoriesService.CreateKey, UsersService.CreateKey and UsersService.CreateSSHSigningKey now take new CreateDeployKeyRequest, CreateUserKeyRequest and CreateSSHSigningKeyRequest (with non-pointer Key) by value instead of *Key.

cc @jvm986 — flagging for #3644 coordination; this covers the Key type (repos/users key endpoints), so it doesn't overlap with #4475.

[gmlewis]

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

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

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.53%. Comparing base (c4176b1) to head (188c6ab).
⚠️ Report is 4 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4477      +/-   ##
==========================================
+ Coverage   98.51%   98.53%   +0.01%     
==========================================
  Files         195      195              
  Lines       17691    17691              
==========================================
+ Hits        17429    17431       +2     
+ Misses        262      260       -2     

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