fix: use non-deprecated LDClient.start with startWaitSeconds

[devin-ai-integration]

Summary

The example called the deprecated LDClient.start(config:context:) overload (deprecated in the iOS SDK: "Use LDClient.start(config: context: startWithSeconds: completion:) to initialize the SDK with a defined timeout"), so building it emitted a deprecation warning. Switched to the supported timeout overload, matching what the sibling examples already do (hello-ios-swift uses startWaitSeconds: 30, hello-tvos uses 10). Also made config a let since it is never mutated (Swift warns otherwise).

-var config = LDConfig(mobileKey: sdkKey, autoEnvAttributes: .enabled)
-LDClient.start(config: config, context: context)
+let config = LDConfig(mobileKey: sdkKey, autoEnvAttributes: .enabled)
+LDClient.start(config: config, context: context, startWaitSeconds: 10)

No SDK version change was needed: Package.resolved already pins ios-client-sdk 11.3.0 and swift-eventsource 3.3.0, which are the latest releases within the configured upToNextMajorVersion 11.0.0 requirement.

Testing

No Xcode on Linux, so this was verified statically against the SDK source at tag 11.3.0 (the deprecation attribute and the replacement signature start(config:context:startWaitSeconds:completion:)). The repo's own CI builds the workspace and the Run workflow launches the app and asserts the window text, which covers the behavior change.

Link to Devin session: https://app.devin.ai/sessions/b10ffc27579444828c87aa46bea06a52
Requested by: @jsonbailey

---

Note

Overview
Updates the hello-macos sample so LaunchDarkly initializes with the non-deprecated LDClient.start(config:context:startWaitSeconds:) API instead of the two-argument overload that triggers a deprecation warning.

LDConfig is now held in a let because it is not mutated, and startup uses a 10 second wait for the client to become ready—aligned with the other platform examples in this repo.

^{Reviewed by Cursor Bugbot for commit 77501b2. Bugbot is set up for automated code reviews on this repo. Configure here.}

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[devin-ai-integration]

@cursor review