feat: add --protocol-era launch flag for ad-hoc servers (#2208)

An ad-hoc --server-url / command launch always negotiated the legacy era,
because protocolEra was only settable per server in mcp.json. A wrapper
that mints a short-lived bearer token therefore had to write that token to
disk to exercise a server under auto or modern.

--protocol-era <legacy|auto|modern> now feeds settings.protocolEra in all
three clients through a shared parseProtocolEra coercer:

- CLI/TUI overlay it in loadServerEntries alongside --header, onto an
  ad-hoc target and onto a --catalog/--config entry for that run.
- Web stamps it on the in-memory ad-hoc entry, which the backend lifts
  into settings like headers; as with --header it is rejected alongside
  --catalog/--config, but unlike --header it applies to stdio too.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01SZ4rrvH6xaCfrst75gJh7x
Signed-off-by: cliffhall <cliff@futurescale.com>

core/mcp/node/config.ts

@@ -8,7 +8,8 @@ import type {
   SseServerConfig,
   StreamableHttpServerConfig,
 } from "../types.js";
-import { normalizeServerType } from "../serverList.js";
+import { isProtocolEra, normalizeServerType } from "../serverList.js";
+import type { ServerProtocolEra } from "../types.js";
 import { toRecord } from "../../json/jsonUtils.js";
 
 /**
@@ -86,6 +87,20 @@ export function parseHeaderPair(
   return { ...previous, [key]: val };
 }
 
+/**
+ * Validate a `--protocol-era` value. Used as the Commander option coerce for
+ * all three clients, so an ad-hoc launch can pick the era without writing an
+ * `mcp.json` entry for it (#2208). Pure function; no Commander dependency.
+ */
+export function parseProtocolEra(value: string): ServerProtocolEra {
+  if (!isProtocolEra(value)) {
+    throw new Error(
+      `Invalid protocol era: ${value}. Valid eras are: legacy, auto, modern`,
+    );
+  }
+  return value;
+}
+
 /** On-disk contents of a freshly seeded empty catalog (pretty-printed). */
 const EMPTY_CATALOG_CONTENT = `${JSON.stringify({ mcpServers: {} }, null, 2)}\n`;