MCP servers, over streamable HTTP

A server is a row with a URL; its tools are discovered by a button and
cached, then offered beside the built-in ones. Written by hand rather than
taken from the reference SDK, because that SDK's transport does its own
connecting -- and the one thing that must not be bypassed is check_url on
every hop. Owning the transport is the point; the framing beside it is the
small part.

Sessions are per call: initialize, initialized, the call, a best-effort
DELETE. Caching one wants an owner, a TTL, eviction, a lock and a shutdown
hook, and the server may expire it under all of that anyway -- ToolContext
is a session-free snapshot precisely so nothing in a tool holds live state.

A server's names and descriptions reach the model as instructions and are
bounded before they do; what it returns is escaped preformatted text, never
markdown. Tools are namespaced per server, so two servers exposing "search"
do not collide and neither shadows a built-in.

Also: a round's calls now run together under a semaphore, results indexed
so each tool turn stays paired with its call, and generation.status names
what is running -- a remote tool is latency-bound, and a silent pause is
what a hang looks like.

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

src/lembas/web/templates/admin/_mcp_row.html

@@ -0,0 +1,36 @@
+{% from "_macros.html" import icon %}
+{#
+  One MCP server in the list.
+
+  Swapped in place by the “Test & refresh” button, so this fragment has to be
+  able to render on its own as well as inside the list.
+#}
+<div class="model-row {{ 'is-off' if not server.enabled }}" id="mcp-{{ server.id }}">
+  <div class="model-row__main">
+    <div class="model-row__title">
+      <a class="model-row__name" href="/admin/mcp/{{ server.id }}/edit">{{ server.name }}</a>
+      <span class="badge">{{ tool_count }} tool{{ '' if tool_count == 1 else 's' }}</span>
+      {% if not server.enabled %}<span class="badge badge--danger">disabled</span>{% endif %}
+      {% if not server.public %}<span class="badge">restricted</span>{% endif %}
+      {% if server.allow_private %}<span class="badge">private network</span>{% endif %}
+      {% if server.protocol_version %}
+      <span class="badge badge--leaf">MCP {{ server.protocol_version }}</span>
+      {% endif %}
+    </div>
+    <code class="model-row__id">{{ server.slug }} · {{ server.url }}</code>
+    {% if message %}
+    <p class="text-xs {{ 'danger' if message_kind == 'error' else 'faint' }}">{{ message }}</p>
+    {% elif server.last_error %}
+    <p class="text-xs danger">{{ server.last_error }}</p>
+    {% endif %}
+  </div>
+
+  <div class="model-row__actions">
+    <button class="btn btn--sm" type="button"
+            hx-post="/admin/mcp/{{ server.id }}/test"
+            hx-target="#mcp-{{ server.id }}" hx-swap="outerHTML">
+      {{ icon("refresh", "icon--sm") }} Test &amp; refresh
+    </button>
+    <a class="btn btn--sm" href="/admin/mcp/{{ server.id }}/edit">Edit</a>
+  </div>
+</div>