test: add integration tests for all backends

[35C4n0r]

• test_policy_resolver: add two tests that exercise a policy_resolver creating fresh TTLPolicy instances per call; the 50-thread scale test reliably reproduces the id()-cache collision that caused wrong threads to be evaluated against the wrong TTL
• test_sweeper: replace test_loop_fails_on_exception (which asserted the broken die-on-error behavior) with test_loop_continues_after_exception and test_loop_logs_exception that verify the loop survives transient failures and emits an ERROR-level log entry
• test_strategy_sqlite: add tests that query the writes table directly after a sweep, and a cross-contamination test that verifies surviving threads are not touched
• test_strategy_postgres: full rewrite - add _row_count helpers, collect tests, hard_age_ttl test, three-table cleanup assertions for both sync and async paths, advisory lock type checks, real two-sweeper lock contention test, and incremental cursor collection test
• pyproject.toml: add test-postgres extra with testcontainers[postgres], langgraph-checkpoint-postgres, and psycopg[binary]
• ci.yml: add test-postgres job that installs test-postgres extra and runs tests/test_strategy_postgres.py; exclude postgres tests from the matrix job so they only run under Docker