refactor: unify channel config matching and gating

Co-authored-by: thewilloftheshadow <thewilloftheshadow@users.noreply.github.com>

extensions/msteams/src/policy.test.ts

@@ -31,6 +31,8 @@ describe("msteams policy", () => {
       expect(res.channelConfig?.requireMention).toBe(true);
       expect(res.allowlistConfigured).toBe(true);
       expect(res.allowed).toBe(true);
+      expect(res.channelMatchKey).toBe("chan456");
+      expect(res.channelMatchSource).toBe("direct");
     });
 
     it("returns undefined configs when teamId is missing", () => {