Python: don't emit ClassVar for a file's top-level extensions (#29222)

Fixes #29221.

### Problem

`PyiGenerator::PrintExtensions` is a template instantiated for two different scopes, and emits `_ClassVar[int]` for both:

| call site | descriptor | output scope | `ClassVar` valid |
| --- | --- | --- | --- |
| `pyi_generator.cc:481` `PrintExtensions(message_descriptor)` | `Descriptor` | class body | yes |
| `pyi_generator.cc:656` `PrintExtensions(*public_dep)` | `FileDescriptor` | module | **no** |
| `pyi_generator.cc:669` `PrintExtensions(*file_)` | `FileDescriptor` | module | **no** |

PEP 526 restricts `ClassVar` to class bodies, so a proto declaring a file-level extension generates a `.pyi` that does not type-check.

Given:

```proto
syntax = "proto2";
package probe;
import "google/protobuf/descriptor.proto";

extend google.protobuf.FileOptions {
  optional bool top_level_ext = 50001;
}

message Holder {
  extend google.protobuf.MessageOptions {
    optional bool nested_ext = 50002;
  }
}
```

before:

```python
DESCRIPTOR: _descriptor.FileDescriptor
TOP_LEVEL_EXT_FIELD_NUMBER: _ClassVar[int]      # module scope
top_level_ext: _descriptor.FieldDescriptor

class Holder(_message.Message):
    __slots__ = ()
    NESTED_EXT_FIELD_NUMBER: _ClassVar[int]     # class body
    nested_ext: _descriptor.FieldDescriptor
    def __init__(self) -> None: ...
```

```
ext_pb2.pyi:7:1 - error: "ClassVar" is not allowed in this context (reportInvalidTypeForm)
```

Only the module-scope constant is flagged; the nested one is correct as-is.

### Change

`PrintEnumValues` in the same file already carries a `bool is_classvar = false` parameter for exactly this distinction. This gives `PrintExtensions` the same parameter and passes `true` only from the message call site, so the two `FileDescriptor` instantiations fall through to a plain `int` annotation. No template specialisation needed.

After:

```python
TOP_LEVEL_EXT_FIELD_NUMBER: int
```

The nested case is unchanged.

### Notes

- Affects any proto with a top-level `extend`, not only extension-only files — the repro above also contains a message.
- `.pyi` golden/expected outputs containing top-level extensions will need regenerating.
- I have not built or run the test suite locally; relying on CI for that.

Closes #29222

COPYBARA_INTEGRATE_REVIEW=https://github.com/protocolbuffers/protobuf/pull/29222 from folded:pyi-no-classvar-for-toplevel-extensions 53f1b7bb99829727d012b0cf5567ff7217d5aaef
FUTURE_COPYBARA_INTEGRATE_REVIEW=https://github.com/protocolbuffers/protobuf/pull/29222 from folded:pyi-no-classvar-for-toplevel-extensions 53f1b7bb99829727d012b0cf5567ff7217d5aaef
PiperOrigin-RevId: 970646537