perf(ssh): bound the system-ssh port-forward stderr tail (#7647)

The stderr handler in system-ssh-port-forward-provider.ts stays attached
for the forward process's entire lifetime and appends every chunk to an
unbounded string, only ever read to build the exit-error detail. A
chatty/warning-spamming remote sshd over a long-lived `ssh -N -L`
forward could grow it without bound.

Fix: keep only the most-recent 64 KB tail, mirroring
MAX_RELAY_STARTUP_BUFFER_BYTES in ssh-relay-deploy-helpers.ts. The tail
is what the error message surfaces anyway.

Test (red->green): emitting >64 KB of stderr then exiting keeps the
recent tail marker and drops the oldest, detail bounded below the
produced size; without the cap the whole string is retained.

src/main/ssh/ssh-port-forward.test.ts

@@ -267,6 +267,27 @@ describe('SshPortForwardManager', () => {
     )
   })
 
+  it('keeps only a bounded tail of a chatty forward stderr (memory-leak regression)', async () => {
+    const onForwardClosed = vi.fn()
+    manager.setCallbacks({ onForwardClosed })
+    const forward = createFakeSystemSshForward()
+    startSystemSshPortForwardProcessMock.mockReturnValue(forward)
+    const conn = createSystemSshConn()
+
+    await manager.addForward('conn-1', conn as never, 3000, '127.0.0.1', 8080)
+    // A long-lived forward against a chatty remote sshd: emit >64 KB of stderr.
+    forward.process.stderr.emit('data', Buffer.from(`HEAD_MARKER${'x'.repeat(70 * 1024)}`))
+    forward.process.stderr.emit('data', Buffer.from('TAIL_MARKER'))
+    forward.process.emit('exit', 255)
+
+    const detail = onForwardClosed.mock.calls[0]?.[1]?.detail as string
+    // The oldest bytes are trimmed; the most-recent tail is retained.
+    expect(detail).toContain('TAIL_MARKER')
+    expect(detail).not.toContain('HEAD_MARKER')
+    // Bounded well below the ~70 KB produced.
+    expect(detail.length).toBeLessThan(66 * 1024)
+  })
+
   it('lists forwards filtered by connectionId', async () => {
     const conn = createMockConn()
     await manager.addForward('conn-1', conn as never, 3000, 'localhost', 8080)

src/main/ssh/system-ssh-port-forward-provider.ts

@@ -33,9 +33,17 @@ export class SystemSshPortForwardProvider implements SshPortForwardProvider {
         )
 
     await forward.waitForStartup()
+    // Why: this stderr stays attached for the forward's whole lifetime but is
+    // only used to build the exit-error detail, so keep a bounded tail — a chatty
+    // remote sshd could otherwise grow it unbounded on a long-lived forward
+    // (mirrors MAX_RELAY_STARTUP_BUFFER_BYTES in ssh-relay-deploy-helpers).
+    const MAX_STDERR_TAIL_BYTES = 64 * 1024
     let stderr = ''
     const onStderr = (chunk: Buffer): void => {
       stderr += chunk.toString('utf-8')
+      if (stderr.length > MAX_STDERR_TAIL_BYTES) {
+        stderr = stderr.slice(-MAX_STDERR_TAIL_BYTES)
+      }
     }
     forward.process.stderr?.on('data', onStderr)