fix(cache): remove same-priority device conflicts

[elezar]

Summary

Make same-priority conflicts for a fully qualified CDI device name unresolvable after cache refresh. This aligns the Rust cache with the Go CDI cache, which removes conflicting names from the newly built local device map before installing it.

The Rust cache currently removes a conflict from self.devices and then overwrites that map with the rebuilt local devices map, leaving the conflict selectable despite refresh() reporting an error.

Related Issue

No issue required: localized cache correctness fix.

Changes

• Remove same-priority conflict names from the rebuilt local device map before assigning it to the cache.
• Assert that a same-priority duplicate reports a refresh conflict and cannot be resolved through get_device.

Downstream motivation

OpenShell currently has a temporary workaround for this cache behavior in NVIDIA/OpenShell#2775. Once it updates to a release containing this fix, that workaround can be removed.

Testing

• cargo test cache::tests
• cargo fmt --check (the pinned Rust 1.94.0 toolchain lacks the rustfmt component in this environment)

Checklist

• Follows Conventional Commits
• Commit is signed off (DCO)

[Copilot]

Pull request overview

This PR fixes a cache correctness bug in Cache::refresh() where same-priority conflicts were reported as errors but could still remain resolvable after the refreshed device map replaced self.devices. The change aligns the Rust cache behavior with the Go CDI cache by ensuring conflicting fully-qualified device names are removed from the newly built local device map before it is installed into the cache.

Changes:

• Remove same-priority conflicting device names from the rebuilt devices map (instead of mutating the soon-to-be-overwritten self.devices).
• Extend the conflict test to assert that get_device() cannot resolve a device name involved in a same-priority conflict.

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