fix(channels): annotate DM messages with sender identity (#120)

* fix(channels): annotate DM messages with sender identity

Telegram and Zalo group messages already include [From: sender] prefix
so the agent knows who is talking, but DM messages were sent without
any sender context — the agent had no way to address the user by name.

- Telegram DM: add [From: @username] (or FirstName if no username)
- Zalo DM: add [From: displayName] when dName is present in payload

* fix(tests): add missing EnsureUserProfile to test stubs

AgentStore interface gained EnsureUserProfile in 4fce731 but the test
stub implementations were not updated, breaking CI on main.

---------

Co-authored-by: Luvu182 <208665161+Luvu182@users.noreply.github.com>

internal/bootstrap/seed_store_test.go

@@ -108,6 +108,9 @@ func (s *seedStubStore) ListUserInstances(_ context.Context, _ uuid.UUID) ([]sto
 func (s *seedStubStore) UpdateUserProfileMetadata(_ context.Context, _ uuid.UUID, _ string, _ map[string]string) error {
 	return nil
 }
+func (s *seedStubStore) EnsureUserProfile(_ context.Context, _ uuid.UUID, _ string) error {
+	return nil
+}
 
 // ---- Tests ----