fix: declare the skills extension in the Inspector's client capabilities (#2373)

SEP-2133 negotiates an extension from both sides, and a strict server
refuses skills/* to a client that did not declare
io.modelcontextprotocol/skills. The Inspector called those methods
without ever declaring it. Add it to ADVERTISABLE_EXTENSIONS, on by
default, which also gives it a Server Settings toggle.

Give the composable test server skillsRequireClientExtension and two
per-era strict showcase configs, so a strict server is reproducible here.

Writing the modern-era test exposed two defects the existing suite hid:
its "modern" cases set protocolEra on the transport config, which
InspectorClient does not read, so they connected on legacy; and once
they really ran modern, every skills result failed, because the modern
schemas required resultType, which the SDK codec checks and then lifts
off before a caller schema runs. Connect with versionNegotiation and
assert the era, and drop the resultType requirement (and the two modern
schema variants it was the only difference in).

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Q3RsrgCxVoFPokLB4vAkxi
Signed-off-by: cliffhall <cliff@futurescale.com>

test-servers/configs/skills-strict-legacy-http.json

@@ -0,0 +1,18 @@
+{
+  "serverInfo": {
+    "name": "skills-strict-legacy",
+    "version": "1.0.0"
+  },
+  "tools": [
+    {
+      "preset": "echo"
+    }
+  ],
+  "resources": [],
+  "skills": true,
+  "skillsRequireClientExtension": true,
+  "transport": {
+    "type": "streamable-http",
+    "port": 3232
+  }
+}

test-servers/configs/skills-strict-modern-http.json

@@ -0,0 +1,19 @@
+{
+  "serverInfo": {
+    "name": "skills-strict-modern",
+    "version": "1.0.0"
+  },
+  "tools": [
+    {
+      "preset": "echo"
+    }
+  ],
+  "resources": [],
+  "skills": true,
+  "skillsRequireClientExtension": true,
+  "transport": {
+    "type": "streamable-http",
+    "port": 3233,
+    "modern": true
+  }
+}