Rust: Make function trait syntax without return type default to unit

[paldepind]

In the special FnOnce(Args) -> ReturnType syntax, the return type can be omitted which is equivalent to specifying () as the return type.

I moved the handling of this from getTypeMentionForTypeParameter to getTypeForTypeParameterAt since we now have a case where a suitable type mention is present.

[Copilot]

Pull request overview

This PR implements proper handling for Rust function trait syntax when the return type is omitted, making it default to the unit type () as per Rust language semantics. This aligns with Rust's specification where FnOnce(Args) is equivalent to FnOnce(Args) -> ().

Changes:

• Added logic to handle omitted return types in function trait syntax, defaulting to unit type
• Moved function trait handling from getTypeMentionForTypeParameter to getTypeForTypeParameterAt to better reflect that a suitable type mention is present
• Added test case to verify the behavior

Reviewed changes

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

File	Description
rust/ql/test/library-tests/type-inference/closure.rs	Added test case for FnOnce(bool) without return type to verify it defaults to ()
rust/ql/test/library-tests/type-inference/type-inference.expected	Updated expected test results with new inferences and shifted line numbers
rust/ql/lib/codeql/rust/internal/TypeMention.qll	Moved and enhanced function trait handling logic to properly support omitted return types

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