Add support for conditionally compiled endpoints

[fruitcoder]

Support conditionally compiled endpoints in @DependencyClient

Previously, @DependencyClient only recognized endpoints declared directly in the struct body. Endpoints wrapped in #if/#endif blocks were silently ignored — no @DependencyEndpoint was applied, and the generated member-wise initializer made no attempt to initialize them.

What changed

DependencyClientMacro now handles IfConfigDeclSyntax during member iteration. Each #if clause is collected into a ConditionalGroup (keyed by its condition string, e.g. os(iOS)), and its properties are processed with the same logic as unconditional members.

Initializer generation is updated to account for conditional endpoints:

• Each conditional group that contains endpoints gets its own #if-wrapped init whose parameters include both the unconditional endpoints and that group's endpoints.
• Every other generated init gets a #if-guarded body block that assigns an unimplemented closure to each conditional endpoint it doesn't take as a parameter, so Swift's definite initialization is satisfied on all platforms.
• The empty/no-endpoint init is always generated, and also contains the conditional unimplemented assignments.

For example:

@DependencyClient
struct Client {
  var fetch: () -> Void
  #if os(iOS)
  var fetchiOS: () -> Void
  #endif
}

expands to three inits:

// Unconditional (always compiled) — unimplemented default for the iOS-only endpoint
init(fetch: @escaping () -> Void) {
  self.fetch = fetch
  #if os(iOS)
  self.fetchiOS = { IssueReporting.reportIssue("Unimplemented: '\(Self.self).fetchiOS'") }
  #endif
}

// iOS-only — takes both endpoints
#if os(iOS)
init(fetch: @escaping () -> Void, fetchiOS: @escaping () -> Void) {
  self.fetch = fetch
  self.fetchiOS = fetchiOS
}
#endif

// No-endpoint init — unimplemented defaults for all endpoints
init() {
  #if os(iOS)
  self.fetchiOS = { IssueReporting.reportIssue("Unimplemented: '\(Self.self).fetchiOS'") }
  #endif
}

Unimplemented closures delegate to DependencyEndpointMacro rather than being hand-rolled. unimplementedAssignment(for:) calls DependencyEndpointMacro.expansion(of:providingPeersOf:in:) and extracts the initializer from the generated _property peer, so the closure is always in sync with what @DependencyEndpoint itself produces (including the \(Self.self) dynamic type name in the issue message).