refactor: extract _collect_fixtures seam from expose

[lesnik512]

What

Splits expose() into a pure decision and a thin installer.

A new private _collect_fixtures(*groups) -> dict[str, AbstractProvider] walks the groups, skips non-Provider attributes, detects cross-group name collisions, and returns a name -> provider mapping. expose() now resolves that mapping (surfacing any error) before touching the module, then installs each provider via the existing modern_di_fixture.

Why

expose() fused three jobs behind one side-effecting interface — discovery, collision detection, and setattr-onto-module. The testable core ("which names, any conflicts?") was only observable past the interface, so the tests had to build throwaway modules and probe with hasattr. And because installation mutates module globals, two configurations couldn't share a module — which is exactly why the suite had four near-identical test_expose_*.py files.

Drawing the seam at the decision makes that core a value you can assert on directly.

Behaviour change: atomic collision

The ValueError is now raised before any setattr, so a failed expose() leaves the target module untouched (previously the first group's fixtures were already installed when the collision fired). Locked in with a test.

Tests: 4 → 3 expose files

• test_collect_fixtures.py (new) — skip / collision / no-groups / mapping rules tested directly against the seam's return value. No throwaway modules, no hasattr probing.
• test_expose.py — integration only: app-scope resolution, multiple groups (folded in), the module= seam, the exec RuntimeError, and the new atomicity guarantee.
• test_expose_request_scope.py — kept separate, with a comment explaining the request_widget name collision that forces it.
• test_modern_di_fixture.py — untouched.

Contract

• Public surface unchanged — still two public symbols; _collect_fixtures is private.
• Error messages verbatim.
• just lint (ruff + ty) clean; just test-ci green at 100% coverage.

🤖 Generated with Claude Code