test: add unit tests for helpers (#815) (#847)

* test: add unit tests for helpers (#815)

Add 63 tests across openai_compatible_helpers, async_helpers, and
server_type covering critical gaps in pure functions used by every
OpenAI-compatible backend.

- extract_model_tool_requests: 9 tests (single/multi/unknown/null
  args/malformed JSON/mixed known+unknown)
- chat_completion_delta_merge: 7 tests (text/reasoning/tool assembly
  with index merging/force-separate/stop_reason)
- message_to_openai_message: 5 tests (text/images/empty images
  list/roles)
- messages_to_docs: 4 tests (empty/single/optional fields/across
  messages)
- send_to_queue: 5 tests (coroutine/async-iter/direct-iter/exception
  propagation)
- wait_for_all_mots: 2 tests (resolve all/empty list)
- get_current_event_loop: 2 tests (running/no-loop)
- ClientCache: 6 tests (put/get/evict-LRU/refresh/overwrite/size)
- _server_type: 7 parametrized tests (localhost variants/openai/
  unknown/malformed URL)

Fixes #815

* refactor: trim 5 trivial/vacuous tests from helpers suite

Remove tests that either test language builtins (OrderedDict.get,
len()), duplicate existing coverage (assistant_role vs text_only),
or can't meaningfully exercise the function under test without a
real pending thunk (wait_for_all_mots).

Author: planetf1

test/helpers/test_async_helpers.py

@@ -0,0 +1,133 @@
+"""Unit tests for mellea.helpers.async_helpers."""
+
+import asyncio
+
+import pytest
+
+from mellea.helpers.async_helpers import (
+    ClientCache,
+    get_current_event_loop,
+    send_to_queue,
+)
+
+# --- send_to_queue ---
+
+
+class TestSendToQueue:
+    async def test_coroutine_single_value(self):
+        """Coroutine returning a non-iterator value is put into queue followed by sentinel."""
+
+        async def produce():
+            return "result"
+
+        q: asyncio.Queue = asyncio.Queue()
+        await send_to_queue(produce(), q)
+        assert await q.get() == "result"
+        assert await q.get() is None  # sentinel
+
+    async def test_coroutine_returning_async_iterator(self):
+        """Coroutine returning an async iterator streams items then sentinel."""
+
+        async def produce():
+            async def _gen():
+                yield "a"
+                yield "b"
+
+            return _gen()
+
+        q: asyncio.Queue = asyncio.Queue()
+        await send_to_queue(produce(), q)
+        assert await q.get() == "a"
+        assert await q.get() == "b"
+        assert await q.get() is None
+
+    async def test_async_iterator_directly(self):
+        """Passing an async iterator (not wrapped in coroutine) streams items."""
+
+        async def _gen():
+            yield 1
+            yield 2
+
+        q: asyncio.Queue = asyncio.Queue()
+        await send_to_queue(_gen(), q)
+        assert await q.get() == 1
+        assert await q.get() == 2
+        assert await q.get() is None
+
+    async def test_exception_propagated_to_queue(self):
+        """Exceptions during generation are put into queue instead of raising."""
+
+        async def explode():
+            raise ValueError("boom")
+
+        q: asyncio.Queue = asyncio.Queue()
+        await send_to_queue(explode(), q)
+        item = await q.get()
+        assert isinstance(item, ValueError)
+        assert str(item) == "boom"
+
+    async def test_iterator_exception_propagated(self):
+        """Exception mid-iteration is captured and put into queue."""
+
+        async def _gen():
+            yield "ok"
+            raise RuntimeError("mid-stream")
+
+        q: asyncio.Queue = asyncio.Queue()
+        await send_to_queue(_gen(), q)
+        assert await q.get() == "ok"
+        item = await q.get()
+        assert isinstance(item, RuntimeError)
+
+
+# --- get_current_event_loop ---
+
+
+class TestGetCurrentEventLoop:
+    async def test_returns_loop_when_running(self):
+        loop = get_current_event_loop()
+        assert loop is not None
+        assert loop is asyncio.get_running_loop()
+
+    def test_returns_none_when_no_loop(self):
+        assert get_current_event_loop() is None
+
+
+# --- ClientCache ---
+
+
+class TestClientCache:
+    def test_put_and_get(self):
+        cache = ClientCache(capacity=3)
+        cache.put(1, "a")
+        assert cache.get(1) == "a"
+
+    def test_evicts_lru(self):
+        cache = ClientCache(capacity=2)
+        cache.put(1, "a")
+        cache.put(2, "b")
+        cache.put(3, "c")  # evicts key 1
+        assert cache.get(1) is None
+        assert cache.get(2) == "b"
+        assert cache.get(3) == "c"
+
+    def test_access_refreshes_lru_order(self):
+        cache = ClientCache(capacity=2)
+        cache.put(1, "a")
+        cache.put(2, "b")
+        cache.get(1)  # refresh key 1 — now key 2 is LRU
+        cache.put(3, "c")  # evicts key 2
+        assert cache.get(1) == "a"
+        assert cache.get(2) is None
+        assert cache.get(3) == "c"
+
+    def test_overwrite_existing_key(self):
+        cache = ClientCache(capacity=2)
+        cache.put(1, "old")
+        cache.put(1, "new")
+        assert cache.get(1) == "new"
+        assert cache.current_size() == 1
+
+
+if __name__ == "__main__":
+    pytest.main([__file__])