Rust: Add predicates for fieldless and unit-only enums

[paldepind]

This PR:

• Changes the isStruct predicates for Struct and Enum. Including unit-like structs/enums in this was inconsistent with Rust terminology.
• Adds predicates to Enum for fieldless and unit-only enums. I think the fieldless notion can be very useful as barriers, similar to what we have for numbers and booleans. A fieldless enum can't cary sensitive data or do injections. I also want to use it over in Rust: Lift content reads as taint steps #20879.

The DCA report is clean 👍

[Copilot]

Pull request overview

This PR refines the CodeQL Rust library's handling of enum and struct predicates to align with Rust terminology and adds new predicates for classifying enum types.

• Changes isStruct() predicates to check for actual struct field lists rather than using negation logic
• Adds isUnit() predicates for structs and variants without field lists
• Introduces isFieldless() and isUnitOnly() predicates for enums with comprehensive test coverage

Reviewed changes

Copilot reviewed 8 out of 9 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
rust/ql/test/library-tests/elements/enum/enums.rs	Adds test data with four enum examples covering unit-only, fieldless, and field-containing enums
rust/ql/test/library-tests/elements/enum/Enum.ql	Adds test queries for the new isFieldless() and isUnitOnly() predicates
rust/ql/test/library-tests/elements/enum/Enum.expected	Defines expected test results showing correct classification of enums
rust/ql/test/library-tests/elements/enum/Cargo.lock	Standard Cargo lock file for test project
rust/ql/lib/codeql/rust/elements/internal/VariantImpl.qll	Refactors isStruct() to check field list type, adds isUnit() and getNumberOfFields() predicates
rust/ql/lib/codeql/rust/elements/internal/StructImpl.qll	Refactors isStruct() to check field list type and adds isUnit() predicate
rust/ql/lib/codeql/rust/elements/internal/EnumImpl.qll	Implements new isFieldless() and isUnitOnly() predicates with proper documentation
rust/ql/lib/codeql/rust/internal/TypeInference.qll	Updates struct/variant declarations to include unit types alongside struct types
rust/ql/lib/codeql/rust/internal/PathResolution.qll	Updates namespace resolution to use isStruct() predicate instead of checking field list type directly

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[hvitved]

LGTM