Resolve short Bluesky handles to bsky.social

[Jeehut]

Picks up the problem from #6 by @n0rthk1n9 without changing the API.

Bluesky handles are domain names: n0rthk1n9.bsky.social for an account on the default server, or a custom domain like fline.dev. LinksKit shares one handle across platforms, so with handle: "Jeehut" for Twitter, Mastodon and Bluesky the Bluesky link pointed at bsky.app/profile/Jeehut, which does not exist.

SocialPlatform.bluesky now normalizes the handle before building the URL:

Handle passed	Link before	Link now
jeehut	bsky.app/profile/jeehut ✘	bsky.app/profile/jeehut.bsky.social
Jeehut	bsky.app/profile/Jeehut ✘	bsky.app/profile/jeehut.bsky.social
@jeehut	bsky.app/profile/@Jeehut ✘	bsky.app/profile/jeehut.bsky.social
jeehut.bsky.social	unchanged	unchanged
fline.dev	unchanged	unchanged

#6 proposed case bluesky(instance: String), analogous to Mastodon. As its author noted in the thread, that breaks custom-domain handles, which have no instance suffix – and it would be a breaking change for everyone using .bluesky. A dot in the handle tells both cases apart reliably, since every full Bluesky handle is a domain.

How this was verified

LinksKit had no tests yet; this adds a LinksKitTests target. SocialPlatformTests covers the five handles above plus @fline.dev, and Mastodon. With the old URL code it failed for the four handles that are short or start with @; on this branch it passes:

# old URL building
✘ blueskyLinksResolveShortAndCustomDomainHandles(handle:expected:) with 6 test cases failed with 4 issues.

# this branch
✔ Test run with 2 tests in 1 suite passed.

jeehut.bsky.social was confirmed to exist through Bluesky's public app.bsky.actor.getProfile API.