[Conformance] Cover the JSON Schema 2020-12 preservation scenario

## Motivation and Context

The `json-schema-2020-12-preservation` client scenario (SEP-1613, SEP-2106; not scored for
2026-07-28, as it was added after the requirements anchor) failed only because the conformance client
had no branch for it: an unknown scenario aborts after connecting, so the harness never observed
a `tools/list` nor the schema echo it diffs.

The client itself already preserves the vocabulary the scenario checks (`$schema`, `$defs` with
`$anchor`, `$ref`, `allOf`/`anyOf`, `if`/`then`/`else`, `additionalProperties`):
`Client::Tool#input_schema` is the parsed `tools/list` response untouched, with no dereferencing,
validation-time rewriting, or keyword stripping, the same fixture-echo approach
the TypeScript and Python conformance clients take. The new branch lists the tools and echoes
the focal tool's observed `inputSchema` verbatim through `json_schema_echo`,
and the expected-failures entry is removed.

## How Has This Been Tested?

The scenario passes 9/9 at its default 2025-11-25 wire. On top of the modern-lifecycle client selection
(the SEP-2322 driver branch) it also passes 9/9 at `--spec-version 2026-07-28`, where the SEP-2106 keyword checks
are failure-gated instead of skipped. `bundle exec rake` is green, with the conformance task passing its baseline check
on both legs, and RuboCop reports no offenses.

## Breaking Changes

None. The change is confined to the conformance client and its baseline; the shipped gem is untouched.

conformance/client.rb

@@ -182,6 +182,14 @@ begin
   case scenario
   when "initialize"
     client.tools
+  when "json-schema-2020-12-preservation"
+    # SEP-1613/SEP-2106: echo the focal tool's observed inputSchema verbatim so the harness
+    # can diff what the client preserved. Preservation holds because `Client::Tool#input_schema`
+    # is the parsed response untouched.
+    tools = client.tools
+    focal = tools.find { |t| t.name == "json_schema_2020_12_tool" }
+    abort("Tool json_schema_2020_12_tool not found") unless focal
+    client.call_tool(name: "json_schema_echo", arguments: { "schema" => focal.input_schema })
   when "tools_call"
     tools = client.tools
     add_numbers = tools.find { |t| t.name == "add_numbers" }

conformance/expected_failures.yml

@@ -8,9 +8,9 @@ client:
   # are not implemented in `MCP::Client::HTTP` yet.
   - http-custom-headers
   - http-invalid-tool-headers
-  # Client-side JSON Schema preservation gaps: `$ref` values must reach the caller unresolved.
+  # Client-side JSON Schema handling: `$ref` values must reach the caller unresolved,
+  # and the conformance client does not run the scenario over the modern lifecycle yet.
   - json-schema-ref-no-deref
-  - json-schema-2020-12-preservation
   # Unimplemented authorization extensions.
   - auth/dpop
   - auth/dpop-nonce