Guard compiler-dependent tests for optional backends

[Copilot]

Compiler-dependent tests should gracefully skip when llvmlite or backend bits aren’t available.

• Test robustness
  • Wrap architecture-agnostic suite with a module-level skip when compiler deps are missing.
  • Use string-based annotations in compiler tests to avoid NameErrors during collection without llvmlite.
• Clarity
  • Keep type expectations while preventing optional dependency import failures.

# Module-level guard
try:
    from eigenscript.compiler.codegen.llvm_backend import LLVMCodeGenerator
except ImportError:
    pytest.skip("Compiler dependencies not installed", allow_module_level=True)

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