Respect comment ignores on proto2 group fields

[lawrence3699]

Fixes #4187

For proto2 group declarations, leading comments are attached to the synthetic nested message source location instead of the group field source location. That meant // buf:lint:ignore FIELD_NOT_REQUIRED on a required group declaration was still ignored even though the same comment worked on a normal required field.

This updates comment-ignore lookup to also check sibling source paths that share the same source span, which covers the field/synthetic-message pair for groups, and adds a regression test for the required-group case.

Validation:

• go test ./private/bufpkg/bufcheck -run 'TestRunComments|TestRunFieldNotRequired(GroupCommentIgnore)?$' -count=1\n- go run ./cmd/buf lint /tmp/buf-group-ignore-eYGbo9 --error-format=json

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