Guard against self-reference in union-type parameter lookup

[lesnik512]

Summary

• For single-type parameters, `Factory._find_dep_provider` already guards against direct self-reference (`if provider is self: return None`). The union-type branch (`for x in v.args:`) had no equivalent guard: it returned the first matching provider regardless of whether that provider was the same `Factory`.
• When the matching provider was `self`, resolution stored `self` in the resolved kwargs and `Factory.resolve` recursed indefinitely on `container.resolve_provider(self)` — `RecursionError` instead of falling through to the parameter's default.
• This change adds `and provider is not self` to the union-loop predicate, mirroring the existing single-type guard.
• One regression test in `tests/providers/test_factory.py` covers the union-with-self-reference case via a creator typed `int | SelfRef = 1`.

Surfaced by the 2026-06-05 bug-hunt audit (`planning/audits/2026-06-05-bug-hunt-audit-report.md`, should-fix-soon #1).

Test plan

• New test `test_factory_self_reference_in_union_falls_through_to_default` passes locally; without the fix it raises `RecursionError`.
• Full suite green, 100% coverage maintained.
• `just lint-ci` green.

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