fix(richtext-lexical): blocksFeature with relationship exposes other tenants (#14985)

Fixes #14823

The issue is essentially the same problem that PR #13229 fixed, but for
a different Lexical feature:

| PR #13229 | PR #14985 (this one) |
| --- | --- |
| Link Feature with internal links | BlocksFeature with relationship
fields inside blocks |
| Relationship to documents exposes other tenants | Relationship inside
blocks exposes other tenants |

## Root Cause

The multi tenant plugin applies tenant filters to relationship fields
using `addFilterOptionsToFields()`, which only runs on collection
fields. However:

- BlocksFeature defines blocks via feature props rather than collection
fields
- These blocks are processed independently through `sanitizeFields()`  
- Relationship fields inside blocks never receive the collection
baseFilter