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>

clients/cli/__tests__/programmatic-ergonomics.test.ts

@@ -227,6 +227,38 @@ describe("CLI --stored-auth-only wiring", () => {
   });
 });
 
+describe("--protocol-era", () => {
+  it.each(["legacy", "auto"])(
+    "accepts %s on an ad-hoc target and connects",
+    async (era) => {
+      const { command, args } = getTestMcpServerCommand();
+      const result = await runCli([
+        command,
+        ...args,
+        "--protocol-era",
+        era,
+        "--method",
+        "tools/list",
+      ]);
+      expectCliSuccess(result);
+    },
+  );
+
+  it("rejects an unknown era before connecting", async () => {
+    const { command, args } = getTestMcpServerCommand();
+    const result = await runCli([
+      command,
+      ...args,
+      "--protocol-era",
+      "future",
+      "--method",
+      "tools/list",
+    ]);
+    expectCliFailure(result);
+    expect(result.stderr).toContain("Invalid protocol era: future");
+  });
+});
+
 describe("MCP_CATALOG_PATH with an ad-hoc target", () => {
   it("does not conflict with an ad-hoc target (env catalog is ignored)", async () => {
     const { command, args } = getTestMcpServerCommand();