Models know how much context they hold

A column rather than a key in capabilities_json, which is rebuilt wholesale
from the submitted checkboxes on every save and would destroy a number
living in it.

0 means unknown, and unknown has to stay tellable from small: the context
percentage and automatic compaction both refuse to act on a figure nobody
supplied. Filled in from /v1/models where the runner advertises it --
OpenRouter, vLLM and llama.cpp each spell it differently, so context_from()
reads the four spellings actually in use, accepts a quoted number but not
"8192 tokens", and rejects anything outside 256..10,000,000. Applied on
discovery only when nothing is set: a refresh must never undo a correction,
since an administrator sets this precisely because the endpoint was wrong.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

tests/test_metrics.py

@@ -0,0 +1,122 @@
+"""How full the context is, what a reply cost, and how fast it arrived."""
+
+from __future__ import annotations
+
+from fastapi.testclient import TestClient
+from sqlalchemy import select
+
+from lembas.db.models import Connection, Model
+from lembas.services.crypto import encrypt
+from lembas.services.llm.openai_client import context_from
+
+
+def _model(db, **kwargs) -> Model:
+    connection = Connection(
+        name="Test", base_url="http://127.0.0.1:1", api_key_encrypted=encrypt("")
+    )
+    db.add(connection)
+    db.commit()
+    model = Model(connection_id=connection.id, model_id="test-model", **kwargs)
+    db.add(model)
+    db.commit()
+    return model
+
+
+# --- Reading a context length off /v1/models ---------------------------------
+def test_context_length_is_read_from_any_of_the_spellings():
+    assert context_from({"id": "m", "context_length": 8192}) == 8192
+    assert context_from({"id": "m", "max_model_len": 32768}) == 32768
+    assert context_from({"id": "m", "context_window": 4096}) == 4096
+    assert context_from({"id": "m", "meta": {"n_ctx": 2048}}) == 2048
+
+
+def test_a_quoted_number_is_accepted_but_a_label_is_not():
+    """Some servers quote it. "8192 tokens" is a label, not a measurement."""
+    assert context_from({"id": "m", "context_length": "8192"}) == 8192
+    assert context_from({"id": "m", "context_length": "8192 tokens"}) == 0
+
+
+def test_an_absent_or_implausible_context_length_is_zero():
+    assert context_from({"id": "m"}) == 0
+    assert context_from({"id": "m", "context_length": 0}) == 0
+    assert context_from({"id": "m", "context_length": 64}) == 0
+    assert context_from({"id": "m", "context_length": 10**9}) == 0
+    # True is an int in Python, and it is not a context length.
+    assert context_from({"id": "m", "context_length": True}) == 0
+
+
+# --- Discovery ---------------------------------------------------------------
+async def test_discovery_fills_in_a_context_length(client: TestClient, db, registered, mock_http):
+    import httpx
+
+    mock_http(
+        lambda _r: httpx.Response(
+            200, json={"data": [{"id": "big-model", "context_length": 16384}]}
+        )
+    )
+    client.post(
+        "/admin/connections",
+        data={"name": "Local", "base_url": "http://x.test", "api_key": ""},
+        follow_redirects=False,
+    )
+    model = db.scalar(select(Model).where(Model.model_id == "big-model"))
+    assert model.context_length == 16384
+
+
+async def test_discovery_never_overwrites_a_number_an_admin_typed(
+    client: TestClient, db, registered, mock_http
+):
+    """A refresh must not undo a correction. Administrators set this precisely
+    because the endpoint was wrong or silent."""
+    import httpx
+
+    mock_http(
+        lambda _r: httpx.Response(200, json={"data": [{"id": "m", "context_length": 4096}]})
+    )
+    client.post(
+        "/admin/connections",
+        data={"name": "Local", "base_url": "http://x.test", "api_key": ""},
+        follow_redirects=False,
+    )
+    model = db.scalar(select(Model).where(Model.model_id == "m"))
+    model.context_length = 131072
+    db.commit()
+
+    connection = db.scalar(select(Connection))
+    client.post(f"/admin/connections/{connection.id}/refresh", follow_redirects=False)
+
+    db.refresh(model)
+    assert model.context_length == 131072
+
+
+# --- The admin field ---------------------------------------------------------
+def test_an_admin_can_set_and_clear_the_context_length(client: TestClient, db, registered):
+    model = _model(db)
+    client.post(
+        f"/admin/models/{model.id}",
+        data={"context_length": "8192", "position": ""},
+        follow_redirects=False,
+    )
+    db.refresh(model)
+    assert model.context_length == 8192
+
+    client.post(
+        f"/admin/models/{model.id}", data={"context_length": "", "position": ""},
+        follow_redirects=False,
+    )
+    db.refresh(model)
+    assert model.context_length == 0
+
+
+def test_junk_in_the_context_length_field_is_ignored_not_a_500(
+    client: TestClient, db, registered
+):
+    model = _model(db, context_length=4096)
+    response = client.post(
+        f"/admin/models/{model.id}",
+        data={"context_length": "eight thousand", "position": ""},
+        follow_redirects=False,
+    )
+    assert response.status_code == 303
+    db.refresh(model)
+    assert model.context_length == 4096