[CI] Matrix conformance jobs over both spec revisions (#458)

* [CI] Matrix the conformance jobs over both spec revisions

Runs server/client conformance against both 2025-11-25 and 2026-07-28,
each against its own endpoint and baseline. PR pipeline pins an exact
conformance package version; conformance-weekly tracks the moving
target on both dist-tags.

* [CI] Run the draft conformance suite from the alpha package

The conformance repo now ships 2026-07-28 scenarios on its alpha
dist-tag, so the draft targets no longer need a local checkout.
Pinned to the same version the pipeline runs.

* [CI] Baseline the failures --suite all newly exposes

The old workflow ran without --suite all/--spec-version, so several
pre-existing gaps never surfaced in CI. Now that both revisions run
the full suite:

- server: json-schema-2020-12 and resources-templates-read are
  fixture bugs shared by both revisions; 2026-07-28 also needs
  resources/subscribe and resources/unsubscribe declared removed
  under the stateless wire, which the server doesn't yet do
- client: the example client doesn't call add_numbers or echo back
  a tool's inputSchema; against 2026-07-28 it doesn't speak the
  lifecycle or OAuth at all yet, so every such scenario fails

Verified locally: all four conformance jobs (server/client x
2025-11-25/2026-07-28) pass against these baselines.

tests/Conformance/conformance-baseline.yml

@@ -1,8 +1,19 @@
-server: []
+# Both pre-existing fixture bugs, not lifecycle-specific: json-schema-2020-12
+# wants a `json_schema_2020_12_tool` fixture neither conformance server
+# defines, and resources-templates-read emits a resource-template URI in a
+# shape the schema rejects. Reproduce identically on the 2026-07-28 endpoint
+# (see conformance-baseline-2026-07-28.yml).
+server:
+  - json-schema-2020-12:json-schema-2020-12-tool-found
+  - resources-templates-read:wire-schema-valid
 
 client:
   - elicitation-sep1034-client-defaults
   - sse-retry
+  # The example client doesn't call add_numbers or echo the tool's inputSchema
+  # back, so these checks have nothing to exercise.
+  - tools_call:tool-add-numbers
+  - json-schema-2020-12-preservation:json-schema-2020-12-client-echo-completed
   - auth/metadata-default
   - auth/metadata-var1
   - auth/metadata-var2