Rust: Refine implSiblings

[hvitved]

#21206 follow-up.

Consider two implementations of the same trait to be siblings when the type being implemented by one is an instantiation of the type being implemented by the other.

For example, in

trait MyTrait<T> { ... }
impl MyTrait<i64> for i64 { ... }    // I1
impl MyTrait<u64> for i64 { ... }    // I2

impl MyTrait<i64> for S<i64> { ... } // I3
impl MyTrait<u64> for S<u64> { ... } // I4
impl MyTrait<bool> for S<T> { ... }  // I5

the pairs (I1, I2), (I3, I5), and (I4, I5) are siblings, but not (I3, I4).

Before this PR, only (I1, I2) were considered siblings.

DCA is fine; while we do loose some call targets, we also fix inconsistencies, most notably we reduce Nodes With Type At Length Limit.

[Copilot]

Pull request overview

Refines Rust trait-impl sibling detection in the type inference / overload resolution logic so that impls are treated as siblings when one Self type is an instantiation of the other, reducing inconsistent resolution outcomes.

Changes:

• Extend shared type inference infrastructure with an isPseudoType hook to ignore pseudo types during non-instantiation checks.
• Update Rust’s implSiblings logic to use instantiation checks rather than syntactic equality of Self type mentions.
• Adjust Rust type-inference regression tests and expected output to match the refined sibling behavior.

Show a summary per file

File	Description
shared/typeinference/codeql/typeinference/internal/TypeInference.qll	Adds isPseudoType to the shared input signature and uses it to ignore pseudo types in isNotInstantiationOf.
rust/ql/lib/codeql/rust/internal/typeinference/TypeInference.qll	Implements isPseudoType for Rust (treating UnknownType/NeverType as pseudo).
rust/ql/lib/codeql/rust/internal/typeinference/FunctionOverloading.qll	Refines implSiblings to consider instantiation relationships between implemented Self types; updates related logic/docs.
rust/ql/test/library-tests/type-inference/regressions.rs	Updates regression annotation to reflect the corrected inferred type after sibling refinement.
rust/ql/test/library-tests/type-inference/type-inference.expected	Updates expected test output to match new inference/resolution results.

Copilot's findings

• Files reviewed: 5/5 changed files
• Comments generated: 1

[Copilot]

In this documentation example, the trait is named MyTrait, but the subsequent impl line uses MyAdd<i64> for i64. This looks like a copy/paste typo and makes the example harder to follow; consider using the same trait name consistently throughout the snippet.

See below for a potential fix:

   * impl MyTrait<i64> for i64 {

[geoffw0]

Results LGTM. Where should I start to understand the implementation?