Part of #1133: clear mypy side-findings

[axisrow]

Part of #1133.

Summary

• Cleared the 8 targeted mypy side-findings from the Part of #1133: clear database union-attr #1208/Part of #1133: annotate collector mixin self types #1209 review without # type: ignore and without touching CI.
• Added explicit Optional guards/asserts where the runtime invariant already existed.
• Used TYPE_CHECKING-only service casts for provider mixin host access and local narrowing for pool_lifecycle / SnapshotCollector paths.

Mypy delta

Check	Before	After
python -m mypy src/	384 errors	365 errors
Exact 8 target file:line reports	8	0
Normalized new errors	n/a	0

The total drop is larger than 8 because the settings guard and photo/bootstrap narrowings also remove adjacent pre-existing reports in the same guarded blocks. Point 7 stayed in this PR; it did not trigger a new-error cascade.

Validation

• ruff check src/ tests/ conftest.py — passed.
• python -c "import src.services.agent_provider_service" — passed.
• python -c "import src.telegram.pool_lifecycle" — passed.
• pytest tests/test_production_limits_service.py tests/test_cli_extended.py tests/test_deepagents_sync_provider_paths.py::TestProviderConfigServiceLoadModelCache tests/test_deepagents_sync_provider_paths.py::TestProviderConfigServiceCompatibility -q — 74 passed.
• pytest tests/routes/test_settings_routes.py tests/routes/test_settings_routes_provider_notifications.py tests/routes/test_photo_loader_routes.py tests/test_bootstrap.py tests/routes/test_accounts_agent_web_mode.py tests/test_web_runtime_shims.py -q — 185 passed.
• pytest tests/test_client_pool_lifecycle.py tests/test_mtproto_watchdog.py tests/test_dialog_cache_ttl.py -q — 41 passed.
• pytest tests/ -k "(provider or pool or bootstrap or photo_loader or production_limits) and not quality" -x -q — 1734 passed, 26 skipped.

Known pre-existing validation note: the exact required broad command pytest tests/ -k "provider or pool or bootstrap or photo_loader or production_limits" -x -q stops on tests/test_quality_scoring_service.py::test_score_content_uses_default_provider. I reproduced the same failure from a clean origin/main worktree at commit 4555c52, so it is not introduced by this branch.