fix(postgres): bind the caller's schema in getTableColumns

[geekypunk]

getTablesAndViews() passes the schema each table was found in, but getTableColumns bound DEFAULT_SCHEMA ('public') and discarded the argument. Every table on a database whose objects live outside public came back with zero columns.

Measured against a real dbt warehouse (195 tables across 17 schemas, nothing in public):

marts.dim_person, schema bound to 'public' ->  0 columns
                                   'marts' -> 91 columns

The PK subquery also had no schema predicate at all. Postgres auto-names primary keys <table>_pkey, so two schemas holding a same-named table cross-match by construction — producing duplicated ColumnInfo rows and false-positive PK flags. This adds tc.table_schema = ku.table_schema and ku.table_schema = ?.

Both problems were latent while the provider only ever read public. #55 made them live: now that introspection walks every non-system schema, a staging/marts/public collision on orders or customers is the normal shape of a dbt warehouse, not the exception.

Verification

Deployed and measured on a live 195-table warehouse:

	before	after
schema snapshot	37 tables / 837 cols	195 / 4,070
tables returning zero columns	195 of 195	0 of 195
tables with correctly-detected PKs	—	67
tables with duplicated column rows	—	0
tables with every column flagged PK	—	0

A full brain re-init on that connection went from a 37-table snapshot to 195 tables / 4,070 columns, lifting table_classification 25 → 161 and inferred_table_relationship 17 → 280. The agent went from being blind to 14 of 18 schemas to correctly describing them.

Unit tests: PostgresIntrospectionProviderTest passes. A mocked ResultSet can't exercise SQL semantics, so the correctness evidence is the measurement above, against Postgres 18.

Relationship to #40

#40 is open and CONFLICTING. It fixed the same class of problem with a current_schema() approach that #55 superseded by scanning all non-system schemas. This PR sits on top of #55 instead and is independent of #40 — #40 can likely be closed once this lands.

[venkateshsakamuri-lab]

Looks good — approve.