Unsupported @callconv forms break AST rendering round trips

The parser accepts unsupported forms of `@callconv` as generic attributes. If a procedure also has a protocol ABI attribute, the parser infers the component model calling convention and stores both forms in the AST.

For example:

```masm
namespace test::callconv

@account_procedure
@callconv("C", "fast")
pub proc foo() -> i1
push.1
end
```

The first parse succeeds. Calling `Module::to_string()` then produces two adjacent attributes:

```masm
@callconv("C", "fast")@callconv("component-model")
```

Parsing the rendered source fails with `conflicting attributes for procedure definition`.

The parser should validate every attribute named `callconv`, regardless of its form. It should reject unsupported forms during the first parse. We should also add a test that parses this input, renders the AST, and parses the result again.

This is related to #3658, which tracks other cases where AST output does not parse.

Look for the parser's attribute validation logic, likely in a module handling procedure definitions or attributes. The test should be added to the existing test suite for parsing/rendering round trips. Start by finding where `@callconv` is currently processed and add validation to reject unsupported forms. Verify by running the new test and ensuring the original example fails to parse.

Do mô hình lập chỉ mục viết ra từ nội dung của issue.