fix(richtext-lexical): preserve field values when blocks are recycled or remounted

[gonzoblasco]

What

Fixes #17232 — In a Lexical BlocksFeature block containing two or more upload fields, selecting a document in one upload field silently erases the other upload field values in that block. The loss is persisted: the next form-state round-trip writes the impoverished state back into the editor node.

Why

When a drawer opens/closes, the BlockComponent is recycled (hide/show via Activity/Offscreen) or hard-remounted. The Form then re-initializes from the stale initialState snapshot captured at block creation time, discarding values adopted since (e.g. uploads set by the user).

The merge from #14295 only iterated cachedFormState entries, so fields absent from the cache (because the cache was built before they were ever set) were silently dropped. This is the key gap: a field like uploadA that had no value when the server built the cache would not appear in cachedFormState, and the merge would never add it back.

How

Three changes to BlockComponent (packages/richtext-lexical/src/features/blocks/client/component/index.tsx):

1. Track the latest adopted form state in a ref

A new latestFormStateRef is updated:

• At the start of onChange (before the server round-trip, capturing the current form state)
• After the server responds with newFormState (capturing the adopted state)
• After the initial client-side getFormState fetch for newly created blocks

This ensures we always have the most recent form state available, regardless of server round-trip timing.

2. Sync the ref into initialState on cleanup and mount

• On cleanup (when the component is hidden/unmounted): setInitialState(latestFormStateRef.current) so that when the component is re-shown, the Form re-initializes from current values instead of the stale creation-time snapshot.
• On mount (covers hard remounts where state is destroyed): restore from the ref if available.

3. Add missing fields from formData during the initial merge

The previous merge from #14295 only iterated cachedFormState entries. Now we also iterate formData and add any fields that are missing from the cache but present in the current block data (with a non-null value). This covers the case where a field had no value when the cache was built server-side but has since been set by the user.

Verification

• Reproduction case from Lexical blocks: selecting an upload wipes other field values in the same block (Form re-initializes from stale initialState) #17232: blocks with 2+ upload fields no longer lose sibling values when selecting uploads
• Pre-existing block functionality (single upload, blocks without uploads) is unaffected
• No new TypeScript errors introduced (all type errors in the file are pre-existing)
• The cacheBuster mechanism for external node.setFields() calls is unchanged

Notes

This addresses all three findings from the root-cause analysis in #17232:

• ✅ Finding 1 (stale initialState on recycle) — fixed by ref tracking + cleanup sync
• ✅ Finding 2 (merge only covers existing cache keys) — fixed by also iterating formData
• ⚠️ Finding 3 (nested group field paths don't match fieldName in formData) — the ref-based approach in fix feat: disable graphQL flag that will bypass gql on payload init #1 largely sidesteps this, since the ref captures the complete form state including nested fields. A dedicated fix for the flat-vs-nested path comparison in the merge would be a follow-up.

Closes #17232