[core] Report missing passthrough shader entry points

[ksh368-bit]

Connections

Fixes #9993.

Description

When a passthrough shader module had no entry points and the pipeline descriptor omitted the entry-point name, the selection logic reported MultipleEntryPointsFound. The same len() != 1 branch handled both empty and ambiguous entry-point sets.

This change separates those cases: an empty set now returns NoEntryPointFound, a single entry point is selected, and multiple entry points continue to return MultipleEntryPointsFound. Explicit entry-point selection keeps its existing behavior.

Testing

• cargo test -p wgpu-core
• cargo test -p wgpu-core --all-features
• cargo clippy -p wgpu-core --tests -- -D warnings
• cargo clippy --tests
• cargo check -p wgpu-core --no-default-features
• cargo fmt --all -- --check
• cargo xtask changelog

Squash or Rebase?

This is a single, logically scoped commit that is ready to rebase onto trunk.

Checklist

• I self-reviewed and fully understand this PR.
• WebGPU implementations built with wgpu may be affected behaviorally.
• Validation and feature gates are in place to confine behavioral changes.
• Tests demonstrate the validation and altered logic works.
• CHANGELOG.md entries for the user-facing effects of this change are present.
• The PR is minimal, and does not make sense to land as multiple PRs.
• Commits are logically scoped and individually reviewable.
• The PR description has enough context to understand the motivation and solution implemented.