fix(terraform): evaluate for expressions in module for_each

[pszypowicz]

Summary

Fixes #7503

When a module's for_each uses a for expression with a simple identity mapping (e.g. { for v in var.items : v => v }), _handle_for_loop_in_dict() assumed iterable items are always dicts and tried obj[k_expression], which raises TypeError on string items. The module was silently skipped.

• Handle the case where the key expression equals the loop variable by using the item directly as the dict key
• Add unit test for the for-expression evaluation
• Add integration test verifying module expansion with a for expression in for_each

Test plan

• New unit test: test_handle_for_loop_in_dict_with_simple_iterable -- {for v in ['a', 'b'] : v => v} evaluates to {'a': 'a', 'b': 'b'}
• New integration test: test_foreach_module_with_for_expression -- module with for_each = { for v in var.items : v => v } expands to child["a"] and child["b"]
• Full tests/terraform/graph/variable_rendering/ suite passes (207 passed, 0 failed)