fix(autogenstudio): preserve DB password in alembic.ini (Fixes #7341) - #8147

Why are these changes needed?

Fixes #7341 — on first-time startup against PostgreSQL, AutoGen Studio's
Alembic migration step (SchemaManager.generate_revision) fails with
password authentication failed even though the app's own engine connects
successfully with the same DATABASE_URI (verified via psql).

Root cause: _generate_alembic_ini_content() serialized the engine URL
with str(self.engine.url). In SQLAlchemy 2.x, str(URL) masks the password
as *** (URL.__str__ renders with hide_password=True). The URL written to
alembic.ini therefore became:

postgresql+psycopg://autogen:***@postgres:5432/autogen

Alembic's autogenerate then opened a fresh connection using the literal
password ***, which PostgreSQL rejects. This only surfaces on first boot
because generate_revision() runs only when the database is empty; on later
restarts the tables already exist and the migration path is skipped, which is
why the UI "works" while the startup log still shows the error.

Fix: render the URL with URL.render_as_string(hide_password=False) so the
real password is preserved in alembic.ini, keeping the existing % → %%
escaping so passwords containing % still survive ConfigParser interpolation.

Related issue number

Closes #7341

Test plan / evidence

• New regression tests in tests/test_schema_manager.py (RED first — both
  failed on main with assert '***' == 'autogen', then passed after the fix):
  • test_alembic_ini_preserves_database_password — the ini URL must contain
    the real password, never ***.
  • test_alembic_ini_roundtrips_password_with_special_characters — a password
    with URL-reserved characters (p@ssw0rd) round-trips correctly through
    URL rendering + ConfigParser %% escaping.
• Local run: pytest tests/test_schema_manager.py tests/test_db_manager.py -q
  → 5 passed; the 3 remaining errors in test_db_manager.py are pre-existing
  on clean main (OpenAIChatCompletionClient fixture fails to construct with
  the released autogen-ext on this host — unrelated to this change, verified
  by running the same tests with the fix stashed).
• ruff check and ruff format --check pass on the changed files.
• Second-agent review: hermes chat -Q fresh-context review of the exact diff
  returned CLEAN / findings handled (see below).
• Note: this is a re-proposal of the same root-cause fix as closed PR fix: preserve database password in Alembic migration config #7349
  (closed unmerged after ~2 months with no maintainer review). This PR adds
  regression tests and an updated explanation; happy to adjust to maintainer
  preference.

Checks

• I've included any doc changes needed for https://microsoft.github.io/autogen/.
• I've added tests (if relevant) corresponding to the changes introduced in this PR.
• I've made sure all auto checks have passed.