RUM-17867: Fixes wrong order setting up session sampler and calling onSessionStart.

When a new RUM session is created, we are doing two things:
1. Copy the session sampler to the `_rumSessionSampler` variable, used by the WebViewTracking feature to obtain sampling information and properly feed it to the JS bridge, which the Browser SDK will use to decide if traces are sampled or not;
2. Call the user provided `onSessionStart` callback if not nil.

Right now, we are calling `onSessionStart` before we copy the sampler to `_rumSessionSampler.` This created a problem: when a WebView is created and a request issued, all at application launch, it’s possible the first requests issued by the page displayed in the WebView are not traced properly since the sampling information is not yet available. Even if the customer used `onSessionStart` to delay the WebView initialization, the sampling information wouldn’t be in place yet when the callback runs, so the problem remained.

This patch inverts the order, so `onSessionStart` will run only after `_rumSessionSampler` contains the session sampler.

Author: arroz

CHANGELOG.md

@@ -1,6 +1,7 @@
 # Unreleased
 
 - [FEATURE] Add support for UK1 Datadog Site. See [#3087][]
+- [FIX] onSessionStart is now called only after sampling information used by WebView Tracking is in place, avoiding missing traces in early requests. See [#3104][]
 
 # 3.14.0 / 15-07-2026
 
@@ -1200,6 +1201,7 @@ Release `2.0` introduces breaking changes. Follow the [Migration Guide](MIGRATIO
 [#3019]: https://github.com/DataDog/dd-sdk-ios/pull/3019
 [#3051]: https://github.com/DataDog/dd-sdk-ios/pull/3051
 [#3087]: https://github.com/DataDog/dd-sdk-ios/pull/3087
+[#3104]: https://github.com/DataDog/dd-sdk-ios/pull/3104
 
 [@00fa9a]: https://github.com/00FA9A
 [@britton-earnin]: https://github.com/Britton-Earnin

DatadogRUM/Sources/Feature/RUMFeature.swift

@@ -111,12 +111,12 @@ internal final class RUMFeature: DatadogRemoteFeature, RUMSessionSamplerProvider
         }()
 
         let onSessionUpdate: RUM.SessionUpdater = { [onSessionStart = configuration.onSessionStart, _rumSessionSampler] sessionScope in
+            _rumSessionSampler.mutate { $0 = sessionScope?.sampler }
             if let sessionScope {
                 let sessionID = sessionScope.sessionUUID.toRUMDataFormat
                 let isDiscarded = !sessionScope.sampler.isSampled
                 onSessionStart?(sessionID, isDiscarded)
             }
-            _rumSessionSampler.mutate { $0 = sessionScope?.sampler }
         }
 
         let dependencies = RUMScopeDependencies(