vector search: hide vector index for sqlite planner

[sivukhin]

Context

PR #1594 tried to solve issue with sqlite planner using vector index for some optimizations (for example in SELECT COUNT(*) FROM t queries).

Turned out hacks to avoid usage of vector indices in planner were not enough. Actually, bUnordered field set by sqlite3 during index analysis process and manipulating with it can be dangerous and error-prone (as sqlite3 can reset it at any moment).

So, this PR fixes this more robustly:

1. opMask is set to 0 for vector index in whereLoopAddBtreeIndex which must additionally prevent sqlite to use index in any operations (>, >=, =, IN) (actually, it shouldn't happen because index has expression - but still - this is another guard)
2. whereIsCoveringIndex return false for vector index
3. Loop over indices in query planner skips vector indices completely
4. All places where optimizer prevented to use bUnordered indices were extended to have additional check for idxIsVector field

[sivukhin]

We currently have some issue and test without db reset starting to generate generic sqlite error in some unexpected places.

This is really weird but I don't understand why it happens and how we can fix it.
Last time I investigated this issue I noticed that SELECT over libsql_vector_meta_shadow table with condition over name field sometimes returns nothing but if you will force full scan of the table - then everything will be fine.

I suspect that issue arise when split of libsql_vector_meta_shadow B-Tree happens. But still not sure why we have this issue...

CC @penberg