Rust: Update split_off models

[geoffw0]

Convert split_off QL-defined barrier to a neutral model (which was always the intent).

Draft: this is atop 7 commits from #21375 , I'll rebase onto main after that is merged.

[Copilot]

Pull request overview

This PR updates the Rust standard-library modeling for split_off by replacing a QL-defined barrier with a models-as-data neutral model, aligning with the earlier intent to represent these “don’t treat as sink” cases in MaD.

Changes:

• Removed the QL-defined barrier that previously suppressed uncontrolled-allocation-size flow into Vec::split_off / String::split_off.
• Added MaD neutralModel entries for Vec::split_off and String::split_off to prevent spurious generated sink models.
• Moved several From::from neutral-model entries for alloc types from core.model.yml into alloc.model.yml.

Reviewed changes

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

File	Description
rust/ql/lib/codeql/rust/security/UncontrolledAllocationSizeExtensions.qll	Removes the dedicated split_off barrier so behavior is controlled via MaD instead.
rust/ql/lib/codeql/rust/frameworks/stdlib/core.model.yml	Removes neutral-model entries for alloc-crate From::from impls (now hosted in alloc.model.yml).
rust/ql/lib/codeql/rust/frameworks/stdlib/alloc.model.yml	Adds neutral models for split_off and relocates neutral models for several alloc From::from impls.

[geoffw0]

Thanks for the quick review, I will wait for DCA and merge (assuming that is uneventful).