Rust: Add barriers for rust/access-invalid-pointer

[paldepind]

This PR is intended to fix false positives for rust/access-invalid-pointer which where exacerbated by #20879.

There's comments in the code, so I hope things make sense.

Note that the change for Default::default could also remove true positives. But right there's just way to many false negatives coming from that.

[Copilot]

Pull request overview

This PR adds barriers and refines sink definitions to reduce false positives in the rust/access-invalid-pointer query. The changes separate concerns between the AccessInvalidPointer and AccessAfterLifetime queries by giving each its own sink definitions with appropriate restrictions.

• Adds type-based barriers (numeric, boolean, fieldless enum) to filter out non-pointer types
• Restricts DereferenceSink in AccessInvalidPointer to only match raw pointer dereferences in unsafe contexts
• Introduces a DefaultBarrier to handle imprecise Default::default calls that can resolve to null pointer implementations

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
rust/ql/src/queries/summary/Stats.qll	Adds import for AccessAfterLifetimeExtensions to ensure all query sink extensions are loaded
rust/ql/lib/codeql/rust/security/AccessInvalidPointerExtensions.qll	Adds multiple barriers (type-based, NonNull::new, Default::default) and restricts DereferenceSink to unsafe contexts with raw pointer type checks
rust/ql/lib/codeql/rust/security/AccessAfterLifetimeExtensions.qll	Refactors Sink from type alias to abstract class and defines its own sink implementations separate from AccessInvalidPointer

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