fix!: remove truncating VariableList conversions and migrate consumers

[storopoli]

Description

• remove From<Vec<T>> for VariableList<T, N> because it silently truncated oversized inputs
• add TryFrom<Vec<T>> for VariableList<T, N> backed by VariableList::new(...)
• update ssz_types decode/docs/tests to use checked construction
• migrate repo-local consumers, benches, and generated code to checked VariableList construction
• refresh ssz_codegen expected outputs to match the new generated code
• preserve infallible view/codegen-owned conversion APIs where length is already validated, using checked construction internally with expect("valid view")

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature/Enhancement (non-breaking change which adds functionality or enhances an existing one)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update
• Refactor
• New or updated tests
• Dependency update
• Security fix

Notes to Reviewers

This is breaking, that's why the last commit bumps the version to 0.15.0

• The main behavioral change is that oversized Vec<T> values can no longer be converted into VariableList<T, N> by truncation.
• Repo-local consumers were migrated to VariableList::try_from(...) or VariableList::new(...).expect("valid view") depending on whether the path is externally fallible or already protected by SSZ/view invariants.
• ssz_codegen generated output changed in many snapshot files because list-to-owned conversion now uses checked construction.
• This is split into two commits:
  • fix(ssz_types): remove truncating VariableList conversion
  • fix(codegen): migrate VariableList consumers to checked constructors

Checklist

• I have performed a self-review of my code.
• I have commented my code where necessary.
• I have updated the documentation if needed.
• My changes do not introduce new warnings.
• I have added tests that prove my changes are effective or that my feature works.
• New and existing tests pass with my changes.

Related Issues

STR-2856 and subtasks from it: STR-2857 and STR-2858

[delbonis]

So now I would kinda expect that new wouldn't take any arguments and would always construct an empty list, since now it's behaviorally duplicating try_from.