fix(retain): make lazy bank creation atomic (#2695)

[ijevin]

Summary

• wrap the dedicated lazy bank-create path in conn.transaction()
• make the bank INSERT and per-bank vector index DDL commit or roll back atomically
• add regression coverage that forces vector-index creation to fail and verifies no bank row survives

Why

The dedicated-connection path in get_or_create_bank_profile() acquired an autocommit connection and then called get_or_create_bank_profile_on_conn(). A new bank row could therefore commit before create_bank_vector_indexes() ran. If that DDL failed, the empty bank remained even though setup did not complete.

This implements the fix described by @benfrank241 in #2695: the existing connection-bound helper stays unchanged, while the wrapper that owns the connection supplies the missing transaction boundary.

Test plan

• uv run pytest tests/test_bank_utils.py -v -n 0
  • verified RED before the production change
  • passes after the transaction wrapper is added
• uv run pytest tests/test_vector_index.py tests/test_bank_utils.py -v -n 0 — 20 passed
• uv run ruff check hindsight_api/engine/retain/bank_utils.py tests/test_bank_utils.py
• uv run ruff format --check hindsight_api/engine/retain/bank_utils.py tests/test_bank_utils.py
• uv run ty check hindsight_api/engine/retain/bank_utils.py
• py_compile for both changed Python files

Environment note

A broader local selection that includes DB-backed hierarchical-config tests could not start because the optional pg0-embedded dependency is not installed in this environment. The focused regression test uses a transaction-aware fake connection so it deterministically exercises the pre-fix autocommit behavior without requiring embedded PostgreSQL.

Fixes #2695