refactor(decorators): move _wrap_task into mixin

[pratyush618]

Summary

Moves _wrap_task (the closure that wraps task functions with hooks, middleware, proxies, and resource injection) and _get_middleware_chain from Queue in py_src/taskito/app.py to QueueDecoratorMixin in py_src/taskito/mixins/decorators.py.

_wrap_task was already invoked from QueueDecoratorMixin.task() (the @queue.task decorator); co-locating it removes the # type: ignore[attr-defined] on that call site and brings app.py from 691 → 577 LOC. The mixin grows by ~140 lines.

The audit recommended mixins/lifecycle.py, but that mixin owns worker startup, banner, and heartbeat — orthogonal concerns. The decorator mixin is the actual consumer of _wrap_task, so it belongs there.

Addresses recommendation 8 from the 2026-05-08 audit (.claude/reports/audit-2026-05-08.md).

Implementation notes

• _emit_event is provided by QueueEventsMixin. The decorator mixin gets a class-level callable annotation (_emit_event: Callable[..., None]) — not a stub method — so type checkers see it without overriding the real implementation through MRO. (An earlier draft used a stub method that silently shadowed the events-mixin implementation and broke workflow event delivery; the regression was caught locally.)
• The full set of Queue attributes that _wrap_task reads (_interceptor, _proxy_registry, _proxy_metrics, _resource_runtime, _test_mode_active, _recipe_signing_key, _max_reconstruction_timeout, _global_middleware) are now declared as class-level annotations on QueueDecoratorMixin.
• app.py drops six imports that were only used by _wrap_task (functools, run_maybe_async, _clear_context, current_job, reconstruct_args, cleanup_proxies, reconstruct_proxies). Those move to the decorator mixin.

Test plan

• cargo test --workspace — clean.
• cargo clippy --workspace — clean.
• uv run python -m pytest tests/python/ — 506 passed, 9 skipped (no regressions).
• uv run mypy py_src/taskito/ tests/python/ --no-incremental — clean.
• uv run ruff check py_src/ tests/ — clean.