fix(aimlapi): send attribution headers on every aimlapi.com request (#17491)

### Summary

The **aimlapi.com** provider added in #17311 does not identify itself on
any of its outgoing requests, so its traffic cannot be attributed to the
integration. This PR adds the two headers AIMLAPI expects —
`X-AIMLAPI-Source` and `X-AIMLAPI-Partner-ID` — to every request the
provider makes.

test/unit_test/common/test_aimlapi_utils.py

@@ -0,0 +1,32 @@
+from common.aimlapi_utils import DEFAULT_PARTNER_ID, DEFAULT_SOURCE, attribution_headers, partner_id, source
+
+
+def test_defaults(monkeypatch):
+    monkeypatch.delenv("AIMLAPI_SOURCE", raising=False)
+    monkeypatch.delenv("AIMLAPI_PARTNER_ID", raising=False)
+
+    assert source() == DEFAULT_SOURCE
+    assert attribution_headers() == {
+        "X-AIMLAPI-Source": DEFAULT_SOURCE,
+        "X-AIMLAPI-Partner-ID": DEFAULT_PARTNER_ID,
+    }
+
+
+def test_environment_overrides(monkeypatch):
+    monkeypatch.setenv("AIMLAPI_SOURCE", " agent/custom ")
+    monkeypatch.setenv("AIMLAPI_PARTNER_ID", " part_custom ")
+
+    assert source() == "agent/custom"
+    assert partner_id() == "part_custom"
+    assert attribution_headers() == {
+        "X-AIMLAPI-Source": "agent/custom",
+        "X-AIMLAPI-Partner-ID": "part_custom",
+    }
+
+
+def test_blank_partner_id_is_omitted(monkeypatch):
+    monkeypatch.delenv("AIMLAPI_SOURCE", raising=False)
+    monkeypatch.setenv("AIMLAPI_PARTNER_ID", "  ")
+
+    headers = attribution_headers()
+    assert headers == {"X-AIMLAPI-Source": DEFAULT_SOURCE}