Simplify FeatureFlagRegistry control flow

[EtaCassiopeia]

Summary

Two small simplifications in FeatureFlagRegistryLive:

1. getClient — fuse .map(...).flatMap(...)

   ZIO#map(f) is implemented as flatMap(a => Exit.succeed(f(a))), so clients.get.map(_.get(domain)).flatMap(...) constructs two FlatMap nodes. Inlining the map into the flatMap callback reduces it to one:

   // Before
   clients.get.map(_.get(domain)).flatMap {
     case Some(c) => ZIO.succeed(c)
     case None    => createDomainClient(domain)
   }
   // After
   clients.get.flatMap(_.get(domain) match {
     case Some(c) => ZIO.succeed(c)
     case None    => createDomainClient(domain)
   })

2. setProvider — collapse two-step for-comprehension

   The for-comprehension introduced an existing binding that was used exactly once. Inlining produces the same effect with less indirection.

Pure refactor; no behavior change.

Credit

Spotted in #110 by @guizmaii.

Test plan

• All tests pass (340 + 99 + 170)
• scalafmtCheckAll clean

[EtaCassiopeia]

Tested independently; closing in favor of #110.