Fix: check ref mut mutability for method call arguments

[Dnreikronos]

Description

Method calls were not checking whether immutable arguments were being passed to ref mut parameters. This check already existed for free/associated function calls in function_application.rs but was missing from method_application.rs.

This PR adds the missing mutability check for method call arguments and fixes pre-existing violations in the standard library and tests that were masked by the absent check.

Closes #7621

Checklist

• I have linked to any relevant issues.
• I have commented my code, particularly in hard-to-understand areas.
• I have updated the documentation where relevant (API docs, the reference, and the Sway book).
• If my change requires substantial documentation changes, I have requested support from the DevRel team
• I have added tests that prove my fix is effective or that my feature works.
• I have added (or requested a) parsing test in core_lang/tests/sway_parse, if relevant.
• I have added the necessary breaking and new feature labels where relevant.
• I have done my best to ensure that my PR adheres to the Fuel Labs Code Review Standards.
• I have requested a review from the relevant team or maintainers.

Changes

Commit 1: Check ref mut mutability for method call arguments

• Added mutability check in method_application.rs that mirrors the existing check in function_application.rs
• Extended gather_mutability() in expression.rs to recurse through StructFieldAccess and TupleElemAccess prefixes, and return Immutable for ConstantExpression
• Skips the implicit self parameter and handles contract call method parameter indexing

Commit 2: Fix immutable arguments passed to ref mut method parameters

• Fixed sway-lib-std/src/hash.sw: Hasher::write now rebinds the immutable bytes parameter to a mutable local before passing to Bytes::append(ref mut other)
• Fixed reexport_paths/src/tests.sw: changed let hasher to let mut hasher before passing to Hash::hash(self, ref mut state: Hasher)

Commit 3: Add E2E test for ref mut method argument mutability check

• New should_fail test covering: immutable variable, struct field access, tuple element access, and constant passed to ref mut method parameter
• Also verifies that mutable variable correctly passes without error

[cursor]

PR Summary

Medium Risk
Tightens type-check semantics for all method calls and may newly fail programs that previously compiled; changes are localized to mutability checking and std/test fixes, not auth or runtime security.

Overview
Method calls now reject immutable actual arguments passed to ref mut parameters, matching the existing check for free/associated function calls.

The compiler extends gather_mutability() so constants, struct field access, and tuple element access are treated as immutable when checking method arguments (skipping self and adjusting indices for contract calls). Hasher::write in std and a reexport test are updated so they pass a mutable binding where ref mut is required. A new should_fail e2e program asserts the Cannot pass immutable argument to mutable parameter diagnostic for several immutable expression forms while allowing a mut variable through.

^{Reviewed by Cursor Bugbot for commit da57ea7. Bugbot is set up for automated code reviews on this repo. Configure here.}