fix: Support SET without spaces around equals sign (#71)

[erans]

Summary

• Fix SET regex: Allow SET DateStyle=ISO (no spaces around =), in addition to the already-supported SET DateStyle = ISO and SET DateStyle TO ISO forms
• Add pg_show_all_settings() support: Rewrite to pg_settings in query preprocessing, routing through existing PgSettingsHandler
• Add set_config() support: Detect set_config(name, value, is_local) calls and return a synthetic wire response with the new value
• Align server_version: Unify all 4 reporting locations (set_handler.rs, state.rs, system_functions.rs, pg_settings.rs) to 16.0

These changes allow pgAdmin4 to complete its initial connection handshake, which sends:

SET DateStyle=ISO; SET client_min_messages=notice; SELECT set_config('bytea_output','hex',false) FROM pg_show_all_settings() WHERE name = 'bytea_output'; SET client_encoding='utf-8';

Closes #71

Test plan

• Unit tests for SET regex: no-spaces, with-spaces, TO keyword, capture verification
• Unit tests for pg_show_all_settings() rewrite (case-insensitive, no-op when absent)
• Unit tests for set_config() detection, captures, empty values, spaces
• Integration test for the full pgAdmin4 query (rewrite + intercept combined flow)
• Full test suite: 485 tests pass, 0 failures
• Pre-commit checklist: cargo check, cargo clippy, cargo build, cargo test

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