fix(fuzz): Do not take a mutable reference over immutable vars which contain a mutable ref

[aakoshh]

Description

Problem*

Resolves one of the errors in https://github.com/noir-lang/noir/actions/runs/15748645365/job/44389551968

Summary*

Fixes the AST generator to consider the mutability of the variable when it generates a target type from a source expression, and avoid taking a mutable reference over the non-mutable parts of an immutable variable, that might also contain a mutable ref.

Additional Context

To reproduce the error:

NOIR_AST_FUZZER_SEED=0x89a06e3200100000 cargo test -p noir_ast_fuzzer_fuzz comptime_vs_brillig 

What happened was that we had a type like this: let d = (false, &mut false); and we were looking to produce an &mut bool; we know d can produce it as d.1, so we chose d as the producer. However, the generator that considered how to produce &mut bool from (bool, &mut bool) thought that it can do &mut <src>.0, not knowing that the source in this case was not mutable.

Documentation*

Check one:

• No documentation needed.
• Documentation included in this PR.
• [For Experimental Features] Documentation to be submitted in a separate PR.

PR Checklist*

• I have tested the changes locally.
• I have formatted the changes with Prettier and/or cargo fmt on default settings.