Upgrade ty to 0.0.69 and fix typing bugs it newly catches

[davegaeddert]

Summary

Bumps the pinned ty type checker from 0.0.65 to 0.0.69 (checked the changelog for 0.0.66–0.0.69) and fixes several latent typing bugs the newer version's more accurate typeshed bundling and stricter checking newly caught — rather than just silencing them with more ignores.

• plain/plain/test/client.py: SimpleCookie[str] was an invalid subscript — SimpleCookie isn't generic in typeshed (it hardcodes BaseCookie[str]). Dropped the subscript.
• plain/plain/utils/datastructures.py: ImmutableList was declared as a plain tuple subclass despite being used as ImmutableList[T] everywhere. Made it properly generic: class ImmutableList[T](tuple[T, ...]).
• plain-postgres/plain/postgres/meta.py / query.py: fixing ImmutableList's genericity made Meta's field-collection properties correctly typed for the first time, which surfaced ~37 further errors from internals code that duck-types across heterogeneous Field subtypes (base Field[T] doesn't declare remote_field/required/clean/etc., which only exist on subclasses, and several call sites rely on runtime type guards ty can't see through). Left those collections as bare ImmutableList rather than forcing a parametrization the field hierarchy doesn't actually support — except related_objects, which is genuinely homogeneous (ForeignObjectRel) and stays precisely typed. Also collapsed a set of @overload stubs in meta.py that had gone dead (all branches now return the same type).
• plain-admin/plain/admin/views/{base,objects,viewsets}.py: AdminViewset.get_views() wired cross-linked URLs onto view classes via dynamic attribute assignment, suppressed with # ty: ignore[unresolved-attribute]. Hoisted the five duplicated URL-getter stub methods (previously copy-pasted across AdminListView/AdminCreateView/AdminDetailView/AdminUpdateView/AdminDeleteView) onto the shared AdminView base class, and typed get_views() against AdminView instead of the generic View, so the assignments type-check for real and the ignore comments are gone.
• plain-admin/plain/admin/cards/base.py: removed one stale, now-unused blanket # type: ignore.
• plain-auth/plain/auth/test.py: the test login_client helper built a cookie-attributes dict that didn't type-check against Morsel.update()'s signature. Rebuilt it on top of Response.set_cookie() — the same path plain.sessions's middleware uses in production — which also fixes a real staleness bug where the test helper had drifted and stopped setting httponly/samesite.

Net effect: type-ignore count across the workspace drops from 160 to 153, with zero new ignores added.

Test plan

• ./scripts/type-validate — 27/27 paths pass, no diagnostics
• ./scripts/fix — ruff check/format clean (oxlint/oxfmt couldn't be reinstalled in this sandbox due to a proxy/TLS limitation unrelated to this change; no non-Python files were touched)
• ./scripts/test (full suite, local Postgres) — all packages pass
• /simplify — ran a 4-angle review (reuse/simplification/efficiency/altitude) over the diff and applied the fixes it surfaced (collapsed dead overloads, restored related_objects's precise type, reused Response.set_cookie() instead of hand-rolling cookie attributes)

Note: I wasn't able to route out-of-scope review findings to PullApprove — the PullApprove5 connector didn't come online in this session. There weren't any left to route in the end; everything the review turned up got fixed.

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