[chain-pr 3/10] DatadogCore: CoreMessageBus implementation and rename

[maxep]

This PR is part of a chain split from maxep/message-bus-subscription by chain-pr.

What and why?

The typed MessageBus protocol introduced in PR #2927 and the test infrastructure added in PR #2928 are ready, but the production MessageBus class in DatadogCore still only implements the legacy FeatureMessage dispatch path. Until CoreMessageBus implements the typed protocol, no production code can route messages through the new typed bus.

This PR is the core implementation layer: it renames the internal MessageBus class to CoreMessageBus to free the protocol name, implements the typed MessageBus protocol on it with a bucket-based dispatch table, wires DatadogCore to expose it via the messageBus property, and migrates the two DatadogContext receivers (ContextSharingTransformer and NetworkContextCoreProvider) from the legacy FeatureMessageReceiver interface to the typed BusMessageReceiver.

How?

MessageBus.swift → CoreMessageBus.swift — typed protocol implementation

CoreMessageBus gains a keyed dispatch table and a full MessageBus conformance:

Addition	Detail
receivers: [String: [ObjectIdentifier: Dispatch]]	Bucket-based registry keyed by BusMessage.key; inner key is the receiver's object identity. Only receivers registered for a given message kind are iterated on dispatch.
subscribe<Receiver>(receiver:)	Registers a typed BusMessageReceiver; captures it strongly in a Dispatch closure. Re-subscribing replaces the previous entry.
unsubscribe<Receiver>(receiver:)	Removes the receiver from its bucket; prunes empty buckets.
send<Message>(message:else:)	Dispatches typed messages to the matching bucket. Intercepts .configuration telemetry for the existing deferred-accumulation path. Calls fallback when the core is nil or the bucket is empty.

Configuration telemetry is still intercepted before bucket dispatch to preserve the existing deferred-accumulation behaviour, and the legacy FeatureMessage path is kept for receivers that haven't migrated yet:

// Typed dispatch to TelemetryMessage subscribers (new)
if let bucket = self.receivers[TelemetryMessage.key] {
    let message = TelemetryMessage.configuration(configuration)
    bucket.values.forEach { dispatch in dispatch(message, core) }
}

// Legacy dispatch for receivers still on the FeatureMessage bus (kept until all migrate)
self.bus.values.forEach {
    $0.receive(message: .telemetry(.configuration(configuration)), from: core)
}

DatadogCore — expose the typed bus

DatadogCore renames its bus property type from MessageBus (the old class, now CoreMessageBus) and exposes the typed bus through DatadogCoreProtocol:

var messageBus: MessageBus { bus }

Context updates are now routed through the typed bus:

// Before
self.contextProvider.publish { [weak self] context in
    self?.send(message: .context(context))
}

// After
self.contextProvider.publish { [weak self] context in
    self?.bus.send(message: context)
}

ContextSharingTransformer and NetworkContextCoreProvider — typed-bus migration

Both DatadogContext receivers are migrated from FeatureMessageReceiver to BusMessageReceiver:

Type	Old	New
ContextSharingTransformer	FeatureMessageReceiver — switches on .context	BusMessageReceiver<DatadogContext> — receives typed context directly
NetworkContextCoreProvider	FeatureMessageReceiver — guards on .context	BusMessageReceiver<DatadogContext> — receives typed context directly

Review checklist

• Feature or bugfix MUST have appropriate tests (unit, integration)
• Make sure each commit and the PR mention the Issue number or JIRA reference
• Add CHANGELOG entry for user facing changes
• Add Objective-C interface for public APIs - see our guidelines (internal)
• Run make api-surface when adding new APIs

---

Chain overview

#	PR	Title
1	#2927	Typed MessageBus protocol and shared message types
2	#2928	Test infrastructure for typed message bus
3	← this PR	DatadogCore: CoreMessageBus implementation and rename
4	—	DatadogCrashReporting: typed-bus migration
5	—	DatadogLogs: typed-bus migration
6	—	DatadogTrace: typed-bus migration
7	—	DatadogFlags: typed-bus migration
8	—	DatadogRUM: typed-bus migration and TelemetryInterceptor merge
9	—	DatadogSessionReplay and DatadogWebViewTracking: typed-bus migration
10	—	Documentation updates

graph LR
    G1["1 — Typed MessageBus protocol and shared message types"]
    G2["2 — Test infrastructure for typed message bus"]
    G3["3 — DatadogCore: CoreMessageBus implementation and rename"]
    G4["4 — DatadogCrashReporting: typed-bus migration"]
    G5["5 — DatadogLogs: typed-bus migration"]
    G6["6 — DatadogTrace: typed-bus migration"]
    G7["7 — DatadogFlags: typed-bus migration"]
    G8["8 — DatadogRUM: typed-bus migration and TelemetryInterceptor merge"]
    G9["9 — DatadogSessionReplay and DatadogWebViewTracking: typed-bus migration"]
    G10["10 — Documentation updates"]
    G1 --> G2
    G2 --> G3
    G3 --> G4
    G3 --> G5
    G5 --> G6
    G3 --> G7
    G6 --> G8
    G7 --> G8
    G8 --> G9
    G1 --> G10

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Merge in dependency order. PRs with no incoming edges from un-merged PRs can be reviewed in parallel.