[kinyoklion]

Summary

The server SDK already had a dependency-aware ChangeNotifier, but applications had no way to reach it. This PR exposes it on Client, adds flag-change-listeners support to the contract test service, and fixes a dependency tracking bug that the new listener tests in sdk-test-harness#514 turned up.

Public API (server SDK only). Adds a new pure virtual to IClient, so any out-of-tree implementation of IClient must now implement it:

virtual IChangeNotifier& FlagNotifier() = 0;   // IChangeNotifier::OnFlagChange(handler) -> unique_ptr<IConnection>

• IDataSystem gains FlagNotifier(). BackgroundSync and FDv2DataSystem return their existing ChangeNotifier. OfflineSystem and LazyLoad return a new NullChangeNotifier, which accepts handlers but never calls them.

Fix: DependencyTracker never updated existing edges. DependencyMap::Get returns a copy and DependencyMap::Set used emplace, which doesn't overwrite. As a result:

• A new dependency on an item that already had dependents was dropped, so the new dependent wasn't notified.
• A removed dependency was never cleared, so the old dependent kept getting notified.

Set now uses insert_or_assign, and UpdateDependencies writes its changes back. The removal path also had the wrong DataKind: it removed (kind_of_dep, key) instead of (kind, key). Unit tests cover all three cases.

Contract tests.

• Adds the flag-change-listeners capability and the registerFlagChangeListener and unregisterListener commands.
• ClientEntity keeps one IConnection per listener ID. The connections are declared after client_, so they disconnect before the client is destroyed.
• The synchronous HTTP POST from ContractTestHook::PostCallback moved into PostJsonCallback so both callers share it.

Verification.

• harness refactor: Implement FDv2 Wire Format + Payload Parser #514 (v3.2.0-alpha.13) with -run 'flag change listeners': 16/16 pass. Before the tracker fix, 14/16 passed; the failures were "a dependency added by an update is followed afterwards" and "a dependency removed by an update is not followed afterwards".
• gtest_launchdarkly-cpp-server: 574/574 pass.

Not in scope: flag-value-change-listeners. The server SDK has no value change listener API, so the harness skips those 16 tests.

Link to Devin session: https://app.devin.ai/sessions/3c3618a040a544a895800896137ad1ca
Open in Devin Desktop: https://app.devin.ai/desktop/session/3c3618a040a544a895800896137ad1ca?variant=devin
Requested by: @kinyoklion

SDK-3355

[devin-ai-integration]

I'll fix CI failures and address comments from users with write access. I'll skip comments containing "(aside)".

• Disable automatic comment, CI, and merge conflict monitoring