PostgresCollection: string/lambda filter produces an invalid WHERE clause (whole predicate collapsed into a string literal)

Describe the bug

PostgresCollection search filters do not work. The generated SQL wraps the entire
filter predicate in a single quoted string literal instead of emitting it as SQL, so
PostgreSQL receives e.g. WHERE '"name" = ''test''' — a text value, not a boolean
predicate — and rejects it (argument of WHERE must be type boolean, not type text).

This affects both string filters and lambda/callable filters, since both flow through
the same _build_filter -> _lambda_parser -> assembly path.

Root cause

_lambda_parser (postgres.py) returns the predicate as a plain Python str
(f-strings), e.g. '"name" = \'test\''. The search query assembly then does
(postgres.py ~L796-801):

if where_clauses := self._build_filter(options.filter):
    query += (
        sql.SQL("WHERE {clause}").format(clause=sql.SQL(" AND ").join(where_clauses))
        if isinstance(where_clauses, list)
        else sql.SQL("WHERE {clause}").format(clause=where_clauses)   # where_clauses is a plain str
    )

psycopg.sql.SQL(...).format(clause=<plain str>) treats a plain str argument as a
literal value, not as SQL, so the pre-built fragment is quoted and its quotes are
doubled. (sql.SQL(" AND ").join([<plain str>, ...]) does the same for the list case.)

Reproduction (offline; no live DB needed to see the malformed SQL)

semantic-kernel 1.44.1, psycopg 3.3.4 (within the pinned psycopg ~= 3.2), Python 3.12:

from dataclasses import dataclass
from typing import Annotated
from psycopg import sql
from semantic_kernel.data.vector import VectorStoreField, vectorstoremodel
from semantic_kernel.connectors.postgres import PostgresCollection

@vectorstoremodel
@dataclass
class Rec:
    id: Annotated[str, VectorStoreField("key")]
    name: Annotated[str, VectorStoreField("data")] = ""
    vector: Annotated[list[float] | None, VectorStoreField("vector", dimensions=2)] = None

col = PostgresCollection(record_type=Rec, collection_name="c")
wc = col._build_filter("lambda x: x.name == 'test'")
print(repr(wc))  # '"name" = \'test\''   (a plain str)
print(sql.SQL("WHERE {clause}").format(clause=wc).as_string(None))
# -> WHERE '"name" = ''test'''      (invalid: the whole predicate is a string literal)

Expected behavior

WHERE "name" = 'test' — the predicate emitted as SQL.

Suggested fix

Mark the pre-built fragment as SQL rather than a value:

clause=sql.SQL(where_clauses)                                   # single
clause=sql.SQL(" AND ").join(sql.SQL(w) for w in where_clauses) # list

Verified this produces the correct WHERE "name" = 'test'.

Security note (so a fix does not regress into injection)

_lambda_parser already escapes string constants (' -> '') and allowlists field
names against the data model, so wrapping the fragment as sql.SQL(...) remains
injection-safe under standard_conforming_strings = on (the PostgreSQL default). If
maintainers prefer, moving values to bound parameters would be more robust than
relying on the manual escaping. I raise this only so the correctness fix is not applied
in a way that turns the currently-collapsed (safe) literal into raw, unescaped SQL.

Notes / limitations

I confirmed the malformed SQL is generated (above); I did not run it against a live
PostgreSQL server, but WHERE '<text>' is rejected by PostgreSQL by design. If there
is a supported configuration where this path works, I'm happy to be corrected.

Start in postgres.py at _build_filter, _lambda_parser, and the search-query assembly around lines 796-801; run the offline psycopg reproduction from the issue to inspect the generated SQL. Done means string and callable filters produce a boolean WHERE predicate rather than one quoted text literal, while preserving the existing escaping and field-name restrictions.