feat: Wire FDv2 connection-mode resolution in common_client

[tanderson-ld]

Summary

Wires the FDv2 mode-resolution scaffolding (merged via #274) into the common_client runtime path. Keeps the FDv1 ConnectionMode enum and all existing public API signatures unchanged.

This is the first half of the further split of the original behavior PR (#275, now closed). The follow-up flutter-only PR wires this into the Flutter SDK.

LDCommonClient

• setMode(ConnectionMode) keeps its FDv1 signature; internally maps the 3 legacy modes to ResolvedConnectionMode before forwarding to DataSourceManager.
• New setResolvedMode(ResolvedConnectionMode) is the advanced FDv2 entry point. Documented as EAP / not-stable. No internal caller in this PR -- the Flutter SDK's ConnectionManager invokes it in the follow-up flutter PR.
• DataSourceFactoriesFn (the public, optional constructor seam) stays typed Map<ConnectionMode, DataSourceFactory> so external callers can still customize streaming + polling. _backgroundFactory is SDK-managed (background is FDv2-only), and _composeFactoriesForManager translates the FDv1 map into the FDv2-keyed Map<FDv2ConnectionMode, DataSourceFactory> consumed by DataSourceManager.

DataSourceManager (internal, not publicly exported)

• Active mode held as ResolvedConnectionMode.
• Factory map keyed by FDv2ConnectionMode.
• ResolvedOffline branches dispatch status as setOffline / networkUnavailable / backgroundDisabled depending on OfflineDetail.

Tests updated to match.

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Note

Medium Risk
Changes core data-source connection and status behavior (including removal of manager-level network toggling); public API is preserved but internal mode transitions and offline semantics are more complex until the Flutter layer adopts setResolvedMode.

Overview
Wires FDv2 connection-mode resolution into the common_client runtime while keeping the public ConnectionMode API unchanged.

DataSourceManager now tracks FDv2ConnectionMode plus OfflineDetail, accepts ResolvedConnectionMode via setMode, and keys factories by FDv2 modes (including background). Offline no longer always means “set offline”: ResolvedOffline maps OfflineSetOffline, OfflineNetworkUnavailable, and OfflineBackgroundDisabled to the matching data-source status without starting a connection. Internal setNetworkAvailable on the manager is removed—network-unavailable behavior is expected to come through resolved offline modes instead.

LDCommonClient maps legacy setMode(ConnectionMode) to resolved modes, adds EAP setResolvedMode, composes FDv1 custom factories into an FDv2 factory map via _composeFactoriesForManager, and registers an SDK-managed background polling factory. setNetworkAvailability no longer touches the data source manager (events only).

Tests are updated for FDv2 modes and the new offline status branches.

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