FN-6808: handle AI CLI probe spawn failures

Resolve synchronous AI CLI probe launch failures as unavailable auth states.

- Catch synchronous spawn failures in Claude and Droid probe helpers so fire-and-forget validation paths do not reject.
- Return unavailable/unauthenticated sentinels for Droid model discovery and CLI presence/auth checks.
- Add regression coverage for Claude and Droid presence/auth probes plus a patch changeset.

Files changed:
 .../fn-6808-cli-probe-unhandled-rejection.md       |  5 ++++
 .../src/__tests__/process-manager.test.ts          | 25 ++++++++++++++++++++
 .../src/__tests__/process-manager.test.ts          | 25 ++++++++++++++++++++
 packages/pi-claude-cli/src/process-manager.ts      | 12 +++++++++-
 .../src/process-manager.ts                         | 27 +++++++++++++++++++---
 5 files changed, 90 insertions(+), 4 deletions(-)

Fusion-Task-Id: FN-6808

Fusion-Task-Lineage: 79702ef2-e116-4313-a557-da17a6888c30

plugins/fusion-plugin-droid-runtime/src/process-manager.ts

@@ -219,10 +219,20 @@ export function captureStderr(proc: ChildProcess): () => string {
  * does this on every chat send), sync probes freeze every other request.
  * This async variant uses spawn so the loop keeps turning while the subprocess
  * starts up.
+ *
+ * FNXC:CliRuntime 2026-06-20-17:25:
+ * FN-6808/FN-6801 require this fire-and-forget auth/presence probe to never reject. Catch synchronous spawn throws from the Vitest child-process guard or platform launch errors and resolve 127, matching the async error sentinel so callers degrade to unauthenticated/not-present instead of surfacing unhandled promise rejections.
  */
 function runDroidProbe(args: string[], timeoutMs = 45000): Promise<number> {
   return new Promise((resolve) => {
-    const proc = spawn("droid", args, { stdio: "ignore" });
+    let proc: ChildProcess;
+    try {
+      proc = spawn("droid", args, { stdio: "ignore" });
+    } catch {
+      resolve(127);
+      return;
+    }
+
     const timer = setTimeout(() => {
       try {
         proc.kill("SIGKILL");
@@ -275,11 +285,22 @@ export async function validateCliAuthAsync(): Promise<boolean> {
 }
 
 export async function discoverDroidModels(): Promise<string[]> {
-  const attempts: string[][] = [["models", "--json"], ["model", "list", "--json"], ["models"]];
+  const attempts: string[][] = [
+    ["models", "--json"],
+    ["model", "list", "--json"],
+    ["models"],
+  ];
 
   for (const args of attempts) {
     const models = await new Promise<string[] | null>((resolve) => {
-      const proc = spawn("droid", args, { stdio: ["ignore", "pipe", "ignore"] });
+      let proc: ChildProcess;
+      try {
+        proc = spawn("droid", args, { stdio: ["ignore", "pipe", "ignore"] });
+      } catch {
+        resolve(null);
+        return;
+      }
+
       let out = "";
       proc.stdout?.on("data", (chunk: Buffer) => {
         out += chunk.toString();