MCP: allow locked exists filters on vector fields declared with index_missing #788

Description

An MCP custom tool profile whose lock.filter uses exists on a vector field is rejected at startup, even when the field is declared with index_missing: true.

_validate_locked_exists_fields (redisvl/mcp/tools/profiles.py) rejects exists on any vector field before it reaches the index_missing check that applies to every other field type. That was the right call while a vector field could not be created with index_missing, because every call would have failed. Once #771 lands, Redis accepts INDEXMISSING on vector fields and answers ismissing(@field) on them, so a locked "only documents that have an embedding" filter becomes a working query that the profile still refuses.

Proposed change

After #771 merges, let exists on a vector field fall through to the existing index_missing check: allow it when the field declares index_missing, and keep the current startup error otherwise. Whether exists on vector fields should be offered in locked filters at all is a design question for the maintainers. If the answer is no, the error message should give that as the reason instead.