feat(memory): add self-hosted PostgreSQL/pgvector memory backend (PgvectorMemory)

[Nantha kumar (Nanduu24)]

Why are these changes needed?

autogen-ext/memory ships chromadb, mem0, redis, and canvas, but none offers semantic vector memory on a general-purpose SQL database the user already controls. Teams running on PostgreSQL that need agent memory on-premise, in air-gapped environments, or under data-residency constraints have no built-in option.

This PR adds PgvectorMemory (autogen_ext.memory.pgvector), a Memory component backed by PostgreSQL with the pgvector extension:

• add embeds a MemoryContent (TEXT / MARKDOWN / JSON) and stores it, with its vector, in a table the user owns (extension, table, and HNSW index created on first use).
• query embeds the query and returns the nearest memories by cosine distance via a pgvector HNSW index, with an optional distance_threshold.
• update_context injects the relevant memories as a SystemMessage, matching the other backends.
• clear / close behave as expected.
• Embeds via a local SentenceTransformers model by default (configurable model_name), with an injectable embedding_function override.
• Ships as the optional extra autogen-ext[pgvector].

It follows the existing redis / chromadb component pattern (a serializable Component config), and the connection pool and embedder are both injectable, which keeps it unit-testable without a live database.

Testing

• Unit tests in tests/memory/test_pgvector_memory.py run against an in-process fake pool and a deterministic embedder (no live DB, driver calls, or network), covering add/query ranking, the distance threshold, JSON round-trips, update_context, clear, the connection-pool lifecycle, and component config serialization — 9 passing.
• Verified end-to-end against a live PostgreSQL 17 + pgvector 0.8.6 instance (real psycopg driver): semantic ranking, JSON round-trip, update_context, cross-instance persistence, the vector column + HNSW index, and clear().
• ruff format, ruff check, and pyright are clean on the changed files.

Related issue number

Feature suggestion: #8306

Checks

• I've included any doc changes needed. The new component is documented via class/config docstrings with a usage example (consistent with the other memory backends); happy to add an API-reference entry if preferred.
• I've added tests corresponding to the changes introduced in this PR.
• I've made sure all auto checks have passed.