feat: FeatherlessClient.embed() against /v1/embeddings (T112.2)

Implements embed() on FeatherlessClient. Featherless's OpenAI-
compatible surface does NOT expose /v1/embeddings at the time of
writing, so this implementation raises NotImplementedError rather
than issuing a request that would 404. The
chat.services.embeddings.generate_embedding wrapper (T112.3)
catches the exception and degrades to the zero-vector fallback path
(plus the existing T107 warning) — misconfigured callers fail loudly
in logs while the request path keeps working.

If/when Featherless ships embeddings, swap the body for
self._client.embeddings.create(model=..., input=...) guarded by
the existing 2-conn semaphore (mirrors generate/stream). The Protocol
seam in T112.1 is already wired so no other code needs to change.

Adds tests/test_featherless.py pinning the NotImplementedError
contract.

chat/llm/featherless.py

@@ -53,3 +53,26 @@ class FeatherlessClient:
                 delta = chunk.choices[0].delta.content or ""
                 if delta:
                     yield delta
+
+    async def embed(self, text: str, *, model: str) -> list[float]:
+        """Embeddings via Featherless — currently unsupported.
+
+        T112 (Phase 4.5) extends the LLMClient Protocol with ``embed()``
+        for a future real-embedding swap. Featherless's OpenAI-compatible
+        surface does NOT expose ``/v1/embeddings`` at the time of writing,
+        so this implementation raises ``NotImplementedError`` rather than
+        attempting a request that would 404. The
+        :func:`chat.services.embeddings.generate_embedding` wrapper
+        catches this and degrades to the existing zero-vector fallback
+        (with the T107 warning), so misconfigured callers fail loudly in
+        logs but the request path keeps working.
+
+        If Featherless ships embeddings, swap the body for an
+        ``self._client.embeddings.create(model=..., input=...)`` call
+        guarded by ``self._sem()`` (mirrors ``generate``/``stream``).
+        """
+        raise NotImplementedError(
+            "Featherless does not expose /v1/embeddings; "
+            "configure a different embedding provider or stick with "
+            "the default pseudo-sha256-384 model."
+        )