[6.x] Deduplicate RelationshipInput in-flight item-data requests

[jasonvarga]

This is one part from #14512

When the RelationshipInput component fires off requests to get item data, if there are multiple fields with the same value they would make identical requests. This could happen when there's a replicator with many of the same repeated subfield.

This change deduplicates concurrent requests so siblings share a single in-flight promise, and keeps that promise alive as long as anyone is still listening.

• Added a module-scope inFlightRequests Map. Each entry holds the shared promise, an AbortController, and a subscriber count. The cache key is a JSON-encoded [itemDataUrl, site, sortedSelections].
• Concurrent requests for the same key reuse the existing entry and increment its subscriber count instead of issuing a fresh request.
• A new detachFromInFlightRequest helper decrements the count on unmount, navigation, or selection change. The shared AbortController only fires when the count reaches zero — so a leader changing selections or unmounting no longer cancels the request out from under followers.
• Each subscriber's .then / .catch / .finally is gated on this._activeRequest === entry, so a component that has moved on to a new selection won't emit stale data or clear loading for an outdated request.
• Map cleanup on settle and on detach are both identity-checked to avoid clobbering a newer entry that happens to share a cache key.