refactor(plugin-postgresql): tidy search-path and centralize identifier/literal escaping

[datlechin]

What

Two related cleanups in the PostgreSQL plugin's SQL generation.

1. Redundant public entry. When the selected schema is public, the search-path builder emitted SET search_path TO "public", public. This collapses it to SET search_path TO "public". Every non-public schema still gets "schema", public, and the quote-doubling injection guard is unchanged.
2. Centralized quoting/escaping. Identifier quoting ("name" with doubled inner quotes) was hand-rolled in ~10 spots across PostgreSQLPluginDriver, RedshiftPluginDriver, and CockroachPluginDriver. Those now call the shared quoteIdentifier(_:) from TableProPluginKit. Duplicated literal escaping (' -> ''), including a fileprivate escapeLiteralForColumns, now routes through the canonical escapeStringLiteral(_:).

Why

A user asked why, after switching to a non-public schema, the SQL editor can still query other schemas unqualified. Investigation confirmed this is correct PostgreSQL behavior, not a bug: search_path is a name-resolution order, not an access boundary, and the , public fallback is the principled choice (dropping it breaks unqualified extension functions/types in public and buys zero security). The only real defect was the redundant "public", public token, plus the scattered copy-paste of escaping logic that the framework already provides in one audited place.

Behavior

• Identifier quoting is byte-identical to the hand-rolled code, so DDL and CREATE/DROP DATABASE output is unchanged.
• escapeStringLiteral also strips null bytes (the old escapeLiteralForColumns did not). This is a defensive improvement with no practical effect: PostgreSQL identifiers cannot contain null bytes.
• PostgreSQLSchemaQueries.setSearchPath keeps its own quoting on purpose. That enum is a standalone, separately unit-tested SQL builder, deliberately decoupled from any driver instance, so it does not depend on the instance quoteIdentifier.

Tests

Added publicSchema() to PostgreSQLSearchPathTests locking in the no-redundancy output. Existing mixed-case, embedded-quote, and injection tests still pass (they take the non-public branch). The quoting/escaping consolidation is behavior-preserving and covered by existing DDL generation paths.

No CHANGELOG or docs entry: no user-facing behavior change.