fix: resolve all clippy warnings for CI (async_fn_in_trait, collapsible_if, type_complexity, dead_code)

- Allow async_fn_in_trait lint crate-wide (internal project, 413 warnings)
- Add integration_tests feature to Cargo.toml to fix unexpected cfg warnings
- Collapse nested if statements into single conditions (13 locations)
- Add type_complexity allows on pg repository functions (12 locations)
- Fix dead code warnings in test modules with allow attributes
- Fix E0599 by gating new_stub() for integration_tests feature
- Add result_unit_err and result_large_err allows where appropriate
- Apply rustfmt formatting

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Cargo.toml

@@ -57,6 +57,7 @@ urlencoding = "2.1.3"
 [features]
 default = []
 test_utils = ["mockall"]
+integration_tests = []
 
 [lints.rust]
 unexpected_cfgs = { level = "warn", check-cfg = ['cfg(integration_tests)'] }