Warn on unclosed Postgres connects in functions

[larimonious]

Summary

• add an ntnt lint warning for function-local std/db/postgres connect() calls that do not also close the handle
• support aliased connect/close imports and wildcard postgres imports
• add CLI lint regressions for unclosed, explicitly closed, and module-scope connects

Verification

• cargo fmt
• cargo build --profile dev-release
• cargo test postgres_connect -- --nocapture
• cargo test

[greptile-apps]

Greptile Summary

This PR adds a lint for function-local Postgres connections. The main changes are:

• Track std/db/postgres connect and close imports, including aliases and wildcard imports.
• Walk function ASTs to warn when a local Postgres connection is not closed.
• Treat branch-local closes as conditional and match closes to specific handles.
• Add lint tests for unclosed, closed, module-scope, conditional, and shadowed connection cases.

Confidence Score: 5/5

This looks safe to merge.

• No blocking issues found in the changed code.

Important Files Changed

Filename	Overview
src/main.rs	Adds AST-based Postgres connect and close linting with alias support, conditional-close handling, and per-handle close matching.
tests/type_checker_tests.rs	Adds tests for the new Postgres connection lint and the previously covered edge cases.

_{Reviews (4): Last reviewed commit: "fix: avoid reusing Postgres close events..." | Re-trigger Greptile}

[larimonious]

Addressed the Greptile findings in f37d25d.

Changes:

• replaced the raw line/brace scanner with AST-based function-body scanning, so strings/comments containing braces cannot hide later connect() calls
• require close() to target the same handle after the connect() line; unrelated closes no longer suppress the warning
• match actual imported call identifiers instead of substring matching, so reconnect()/my_connect() do not false-positive
• added regression tests for all three cases

Verification:

• cargo fmt
• cargo build --profile dev-release
• cargo test test_lint -- --nocapture
• cargo test
• git diff --check