[FTR] Configure undici timeouts on KbnClient dispatcher (elastic#270932)

## Summary

Set `connect.timeout = 60s` on the undici `Agent` used by
`KbnClientRequester` (https path only).

## Why

elastic#268531 migrated `KbnClient` from axios to native fetch but did not
override undici's 10s `connect.timeout` default. Axios had no equivalent
cutoff, so FTR callers talking to a busy local Kibana started failing
once that PR landed.

The `kibana-streams-performance` weekly pipeline went red in builds #9,
#11, #12, and #13 with:

```
ConnectTimeoutError: Connect Timeout Error (attempted address: localhost:5620, timeout: 10000ms)
```

The `10000ms` is undici's default. Bisect: build #8 last green
(2026-05-11) → #9 first red (2026-05-18), with elastic#268531 in the window.

## What changed


`src/platform/packages/shared/kbn-kbn-client/src/kbn_client/kbn_client_requester.ts`:
one constant, one option on the https `Agent`. http branch unchanged.

## Related

Regression introduced in elastic#268531. Companion streams perf PR: elastic#270636.

## Validation

https://buildkite.com/elastic/kibana-streams-performance/builds/14

Author: rStelmach

src/platform/packages/shared/kbn-kbn-client/src/kbn_client/kbn_client_requester.ts

@@ -118,6 +118,12 @@ interface Options {
   certificateAuthorities?: Buffer[];
 }
 
+// undici's default `connect.timeout` is 10s. FTR talks to a local Kibana that
+// can take longer to accept a connection during heavy load (e.g. while
+// streaming a large multipart upload), and the previous axios-based requester
+// had no equivalent socket-connect cutoff. Restore that headroom.
+const FETCH_CONNECT_TIMEOUT_MS = 60_000;
+
 export class KbnClientRequester {
   // `url` retains any `user:pass@` from the original config - `resolveUrl()` is
   // a public API used by FTR tests (e.g. http connector tests) that pluck
@@ -145,7 +151,13 @@ export class KbnClientRequester {
 
     this.dispatcher =
       parsed.protocol === 'https:'
-        ? new Agent({ connect: { ca: options.certificateAuthorities, rejectUnauthorized: false } })
+        ? new Agent({
+            connect: {
+              ca: options.certificateAuthorities,
+              rejectUnauthorized: false,
+              timeout: FETCH_CONNECT_TIMEOUT_MS,
+            },
+          })
         : null;
   }