Run the extension-tagged conformance scenarios in CI as baselined known failures

The tasks-* scenarios are extension-tagged: they never match a
--spec-version filter and sit on the harness's pending list, so none of
the existing server legs can select them — they were silently absent
from CI with no skip line and no waiver. server-sse-polling (pending,
removed at 2026-07-28) was equally unreachable.

Add a bare --suite all server leg, which selects every scenario shipped
with the pinned harness, and baseline the nine runnable tasks-*
scenarios in expected-failures.yml. The stale-entry rule turns the
baseline into a forced burn-down list as tasks support lands.
server-sse-polling turns out to already pass. tasks-status-notifications
is deliberately not baselined: the harness skips it unconditionally, and
a baseline entry for a scenario with no failing checks is itself flagged
stale.

Author: maxisbey

.github/actions/conformance/expected-failures.yml

@@ -12,4 +12,23 @@
 
 client: []
 
-server: []
+server:
+  # SEP-2663 (io.modelcontextprotocol/tasks): the SDK does not implement the
+  # tasks extension yet. These extension-tagged scenarios are selected only by
+  # the bare `--suite all` leg — extension scenarios never match a
+  # --spec-version filter and the active/draft suites exclude them — so these
+  # entries are inert for the other legs that read this file.
+  #
+  # `tasks-status-notifications` is intentionally NOT listed: the harness
+  # skips it unconditionally (pending its rewrite against subscriptions/
+  # listen), and a baseline entry for a scenario with no failing checks is
+  # flagged stale.
+  - tasks-lifecycle
+  - tasks-capability-negotiation
+  - tasks-wire-fields
+  - tasks-request-state-removal
+  - tasks-mrtr-input
+  - tasks-request-headers
+  - tasks-dispatch-and-envelope
+  - tasks-required-task-error
+  - tasks-mrtr-composition

.github/workflows/conformance.yml

@@ -75,6 +75,22 @@ jobs:
           --suite all
           --spec-version 2026-07-28
           --expected-failures ./.github/actions/conformance/expected-failures.2026-07-28.yml
+      - name: Run server conformance (all suite, extension scenarios)
+        # A bare `--suite all` (no --spec-version) selects every scenario
+        # shipped with the pinned harness — including the extension-tagged
+        # tasks-* scenarios and pending-listed ones like server-sse-polling,
+        # which no other leg reaches (extension scenarios never match a
+        # --spec-version filter, and the pending list keeps them out of the
+        # active suite). Running the full set keeps unimplemented surfaces
+        # visible as baselined known failures in expected-failures.yml instead
+        # of silent exclusions, and stays robust to scenarios moving between
+        # harness suite lists across pin bumps. `--suite pending` would cover
+        # the same union slightly faster; the full set is preferred for the
+        # self-contained run and for parity with typescript-sdk's CI.
+        run: >-
+          ./.github/actions/conformance/run-server.sh
+          --suite all
+          --expected-failures ./.github/actions/conformance/expected-failures.yml
 
   client-conformance:
     runs-on: ubuntu-latest