fix: relax pgvector upper bound to <0.5

[paulflew]

Problem

The pgvector dependency is pinned to >=0.2.5,<0.4. This upper bound prevents installing langchain-postgres alongside packages that require pgvector>=0.4.2 (e.g. lightrag-hku), causing unresolvable dependency conflicts.

Fixes #307.

Change

Relax the upper bound to >=0.2.5,<0.5, allowing the pgvector 0.4.x series while staying conservative (does not open up to an untested 0.5+).

Why this is safe

The only pgvector API surface used by this package is pgvector.sqlalchemy.Vector (langchain_postgres/vectorstores.py), which is unchanged across the 0.2.x–0.4.x line.

Verification

• uv lock resolves cleanly to pgvector==0.4.2 under the new constraint.
• uv sync --group test installs against 0.4.2, and from pgvector.sqlalchemy import Vector, import langchain_postgres, and from langchain_postgres import PGVector, PGVectorStore all import successfully.
• uv.lock is regenerated and committed to stay in sync.

[paulflew]

Verified locally against the version this change unblocks:

• uv lock resolves cleanly to pgvector==0.4.2 under the new >=0.2.5,<0.5 constraint.
• Full test suite run against a live pgvector/pgvector:pg16 Postgres with pgvector==0.4.2 installed: 749 passed, 25 skipped, 0 failed (v1 + v2, sync + async, including the langchain_tests standard vectorstore conformance suite).

The only pgvector API used is pgvector.sqlalchemy.Vector, which is unchanged across the 0.2.x–0.4.x line, so the bump is safe.