Use includes_for_codegen to filter annotations

Summary: Addresses reviewer feedback (hchok) on D94439428 — removes the fragile `is_annotation?` string-contains check (`ns.find("Annotation")`) and replaces it with a `codegen_includes` property on the `t_program` prototype that uses the canonical `t_program::get_includes_for_codegen()` method. This is consistent with how other Thrift generators filter annotation includes.

Reviewed By: vitaut

Differential Revision: D94711724

fbshipit-source-id: cfb7af3af656c5c8b9d34dae345fc1c77b90b968

Author: abrady

third-party/thrift/src/thrift/compiler/generate/templates/csharp/module.mustache

@@ -16,12 +16,9 @@
 }}
 {{#import "types/enum" as enum_tmpl}}
 {{> common/file_header}}
-{{#each root_program.includes as |inc|}}
+{{#each root_program.includes_for_codegen as |inc|}}
 {{#if inc.is_useful_alias?}}
-{{! Skip annotation includes - they are compile-time only metadata, not runtime types }}
-{{#if (not inc.is_annotation?)}}
 using {{inc.alias}} = {{inc.csharp_namespace}};
-{{/if (not inc.is_annotation?)}}
 {{/if inc.is_useful_alias?}}
 {{/each}}