Python: don't emit ClassVar for a file's top-level extensions

[folded]

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:

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:

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:

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.