fix(pgwire): bypass cache for data-dependent point plans - #185

Summary

• classify document point reads and PK-targeted mutations as data-dependent plans
• keep mutable PK-to-surrogate bindings out of the schema-only physical-plan cache
• cover simple and extended query reads plus point and multi-key UPDATE/DELETE regressions

Validation

• cargo nextest run -p nodedb --test point_get_after_miss_then_insert
• cargo nextest run -p nodedb-sql -E 'test(cacheability)'\n- cargo clippy -p nodedb -p nodedb-sql --all-targets -- -D warnings\n- cargo fmt --all --check