feat(typecheck): typecheck CREATE … AS query bodies

[edjubert]

What kind of change does this PR introduce?

Feature.

What is the current behavior?

Typechecking skips the query body of CREATE TABLE AS, CREATE MATERIALIZED VIEW AS,
and CREATE VIEW AS.

As a result, database errors inside the embedded SELECT are not reported.

Fixes #786.

What is the new behavior?

The typechecker now extracts and prepares the inner query of supported CREATE … AS
statements without executing the outer DDL.

PostgreSQL diagnostics are mapped back to their original positions in the enclosing
SQL document. Valid create-as statements produce no diagnostic and do not create the
target table or view.

Additional context

Added regression coverage for:

• invalid columns and relations in CREATE TABLE AS;
• invalid columns in materialized views and views;
• valid create-as queries without DDL side effects;
• workspace-level file-relative diagnostic offsets.

Validated with:

cargo test -p pgls_typecheck
cargo test -p pgls_workspace test_create_as_typecheck_diagnostic_offsets
cargo fmt --all -- --check
cargo clippy --all-targets --all-features -- -D warnings