Fix ref without mut parameter check on methods and associated functions

[Dnreikronos]

Description

Fixes #7623.

ref without mut on function parameters was incorrectly handled:

• On free functions, it produced a confusing call-site error ("Cannot pass immutable argument to mutable parameter") because ref alone was mapped to RefMutable, which is_mutable() treats as mutable.
• On methods and associated functions, it silently worked because method calls don't perform the same call-site mutability check.

Root cause: VariableMutability::new_from_ref_mut(is_reference=true, is_mutable=false) returned RefMutable — treating an immutable reference as mutable.

Changes

• Add RefImmutable variant to VariableMutability for ref without mut parameters
• Fix new_from_ref_mut to correctly distinguish ref mut (RefMutable) from ref alone (RefImmutable)
• Update call-site mutability check to only require mutable arguments for ref mut parameters
• Add display support for RefImmutable variant

Behavior after fix

Scenario	Result
ref param accepts immutable arg (free fn)	Allowed
ref param accepts immutable arg (method/associated fn)	Allowed
ref param is immutable inside function body	Enforced
ref mut param still rejects immutable arg	Enforced

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 (Forc, Sway book, etc.).
• 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/trybuild-tests if needed.
• I have added the necessary Breaking* or 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.

[cursor]

PR Summary

Medium Risk
Changes core parameter mutability classification and function-call checking, which affects all ref/ref mut call sites but is narrowly scoped with targeted e2e tests.

Overview
Fixes incorrect typing of ref parameters (without mut) by introducing VariableMutability::RefImmutable and correcting new_from_ref_mut so reference-only params are no longer classified as RefMutable.

Call-site checking in function application now requires a mutable argument only when the parameter is ref mut (is_reference && is_mutable), not for plain ref, so immutable values can be passed consistently for free functions, methods, and associated functions. Declaration pretty-printing includes the new ref case.

E2E tests cover accepting immutable args with ref params and rejecting assignment inside the callee and ref mut with immutable arguments.

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