test: check query validation

[tisonkun]

This closes rust-lang/rust-clippy#10

[Copilot]

Pull request overview

Adds PostgreSQL syntax validation to the pqb integration test suite so that snapshot-tested SQL is parsed/validated (closing the gap described in #10: “Validate syntax of generated SQL”).

Changes:

• Introduce a ValidateSQL helper trait in pqb/tests/common.rs that parses SQL with pg_query.
• Update most SQL snapshot assertions across test modules to call .validate() on the generated SQL.
• Add pg_query as a dev-dependency (and update Cargo.lock accordingly).

Reviewed changes

Copilot reviewed 10 out of 11 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
pqb/tests/common.rs	Adds ValidateSQL helper that parses SQL via pg_query.
pqb/tests/select.rs	Applies .validate() to generated SELECT SQL snapshots (and a couple of statement snapshots).
pqb/tests/insert.rs	Applies .validate() to INSERT snapshots; leaves one known-invalid ON CONFLICT case unvalidated with a FIXME.
pqb/tests/create_table.rs	Applies .validate() to CREATE TABLE snapshots; leaves generated-column case unvalidated due to dialect/version mismatch.
pqb/tests/index.rs	Applies .validate() to CREATE INDEX snapshots.
pqb/tests/drop.rs	Applies .validate() to DROP statements.
pqb/tests/explain.rs	Applies .validate() to EXPLAIN statements.
pqb/tests/expr.rs	Applies .validate() to expression-driven SELECT snapshots.
pqb/tests/types.rs	Applies .validate() to identifier/qualified name rendering snapshots.
pqb/Cargo.toml	Adds pg_query = 6.1.1 under dev-dependencies.
Cargo.lock	Locks the new dependency graph introduced by pg_query.

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[tisonkun]

Waiting for pganalyze/libpg_query#304

[tisonkun]

Clippy false positive - https://github.com/rust-lang/rust-clippy/issues/16534