fix(types): relax type constraints broken by stripInternal

[YevheniiKotyrlo]

Summary

Two fixes for type declarations that become invalid after @internal properties are stripped from .d.ts output by stripInternal: true:

1. Relax SelectWithout generic constraint (3 files)

K extends keyof T & string → K extends string in:

• src/mysql-core/query-builders/select.types.ts
• src/singlestore-core/query-builders/select.types.ts
• src/sqlite-core/query-builders/select.types.ts

The constraint fails because set operator excluded method lists include 'session' — a property marked @internal and stripped from .d.ts. When 'session' is absent from the emitted type, 'session' no longer satisfies keyof T & string. Omit<> already handles missing keys safely (they're simply ignored), so K extends string is sufficient.

2. Remove implements clause from Policy classes (2 files)

PgPolicy implements PgPolicyConfig → PgPolicy in:

• src/pg-core/policies.ts
• src/gel-core/policies.ts

The config interfaces have optional properties (as?, for?), but the class declares them as non-optional readonly. After @internal stripping, the implements clause produces TS2420 because the structural match fails. The class remains structurally compatible without the clause — removing it eliminates the false type error without changing behavior.

Related: #5187, #4299