sem/tree: mark LTREE as ambiguous type for proper casting

LTREE datums need explicit type casts when formatted to avoid being
misinterpreted as strings by the query engine. Previously, LTREE[]
arrays were formatted as ARRAY['j.k.l'] which loses type information,
causing the query engine to interpret them as string[] and resulting
in errors like:
```
unsupported comparison operator: <ltree[]> >= <string[]>
```

Marking LTREE as an ambiguous type ensures the formatter includes
explicit type casts (e.g., ARRAY['j.k.l']::LTREE[]), preventing type
mismatches in distributed queries.

Closes: #156121
Epic: CRDB-55075

Release note: none

Author: spilchen

pkg/sql/sem/tree/datum.go

@@ -4273,7 +4273,7 @@ func (d *DLTree) Max(ctx context.Context, cmpCtx CompareContext) (Datum, bool) {
 }
 
 // AmbiguousFormat implements the Datum interface.
-func (*DLTree) AmbiguousFormat() bool { return false }
+func (*DLTree) AmbiguousFormat() bool { return true }
 
 // Format implements the NodeFormatter interface.
 func (d *DLTree) Format(ctx *FmtCtx) {