Add modern-era (2026-07-28) test-server preset (#1700) (#1703)

* Add modern-era (2026-07-28) test-server preset (#1700)

The era-aware connection model (#1626, PR #1697) added per-server
`protocolEra` and the era-aware Connection Info UI, but no bundled test
server negotiated the modern era — the modern happy path had only
unit/Storybook fixtures, no live coverage.

Serve the modern (2026-07-28) protocol era from the HTTP test server via
the SDK's `createMcpHandler`. A `modern` flag on `ServerConfig` (and a
`transport.modern` field in the JSON config) mounts the handler with a
dual-era `legacy: "stateless"` fallback by default, or `legacy: "reject"`
for modern-only strict serving. The web-standard `fetch` handler reuses
the existing `toWebRequest`/`writeWebResponse` conversion, so no new
`@modelcontextprotocol/node` dependency is needed.

An SDK v2 client negotiating `protocolEra: "auto" | "modern"` now reaches
the modern leg end-to-end: `getProtocolEra() === "modern"`,
`getNegotiatedProtocolVersion() === "2026-07-28"`, and a populated
`getDiscoverResult()`. Verified live in the Inspector — Connection Info
shows Era Modern / Session Sessionless / the Discovery section.

- Modern serving path + `listen`/`startModernHttp` helpers in
  test-server-http.ts; modern-leg teardown in stop().
- `modern` config field threaded through load-config/resolve-config with
  a streamable-http-only validation guard; modern-http.json demo config.
- Integration test covering auto/modern negotiation, the dual-era legacy
  fallback, a modern tools/call, and the strict `legacy: "reject"`
  rejection.

Closes #1700

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01XNdjEPKLG637X8YmhDiEk5

* Review nit: gate transport.modern guard on truthiness

A falsy `transport.modern` (e.g. `false`) is a no-op that `resolveConfig`
normalizes to `undefined`, so it shouldn't trip the streamable-http-only
guard. Gate on truthiness instead of `!== undefined` so only *enabling*
modern on a non-HTTP transport is rejected.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01XNdjEPKLG637X8YmhDiEk5

---------

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

test-servers/configs/modern-http.json

@@ -0,0 +1,12 @@
+{
+  "serverInfo": {
+    "name": "composable-modern",
+    "version": "1.0.0"
+  },
+  "tools": [{ "preset": "echo" }, { "preset": "get_temp" }],
+  "transport": {
+    "type": "streamable-http",
+    "port": 3100,
+    "modern": true
+  }
+}