feat: add Group.get_named_providers()

[lesnik512]

What

Adds Group.get_named_providers() -> dict[str, AbstractProvider] — an MRO-walking accessor that maps each declared attribute name to its provider — and reimplements Group.get_providers() as list(cls.get_named_providers().values()), so the traversal and dedup/masking logic lives in one place.

Why

get_providers() discards the attribute name each provider was declared under. Downstream integrations that need names (notably modern-di-litestar's autowiring) currently reconstruct them with a fragile id()-keyed reverse lookup over group.__dict__. That lookup only sees the subclass __dict__ while get_providers() walks the full MRO, so autowiring a Group that inherits a provider raises KeyError. Exposing names at the source — where Group owns provider declaration and traversal — fixes the bug for every consumer.

The downstream modern-di-litestar change consumes this and is a separate PR, gated on the 2.20.0 release of this package.

Behavior

get_named_providers() preserves the exact semantics of the old get_providers() traversal:

• MRO order (most-derived first), now pinned by a test
• first-seen name wins (diamond inheritance returns each provider once)
• a non-provider override masks the parent provider of the same name

get_providers()'s contract (return type, order, dedup, masking) is unchanged — proven by the pre-existing tests plus a consistency test.

Tests / checks

• just test-ci: 100% line coverage, 236 passed
• ruff format / ruff check / ty: clean
• architecture/providers.md promoted in-PR; just docs-build clean
• planning bundle added (planning/changes/2026-06-25.04-group-named-providers/); just check-planning OK

After merge

Cut tag 2.20.0 to publish, which unblocks the modern-di-litestar autowiring PR.