fix: spell source NUL separators as Unicode escapes (#26864)

Co-authored-by: Matheus Breguêz <23284276+matbrgz@users.noreply.github.com>

cloud/apps/relay/src/idle-regional-rehome-selection.ts

@@ -300,7 +300,7 @@ function inlineRows(columns: ReadonlyArray<readonly [string, string]>, rows: num
 }
 
 function hostKey(row: SqlRow): string {
-  return `${String(row.user_id)}${String(row.relay_host_id)}`
+  return `${String(row.user_id)}\u0000${String(row.relay_host_id)}`
 }
 
 function idleRehomeCandidate(

src/main/runtime/browser-client-download-transfers.ts

@@ -330,5 +330,5 @@ function isDestinationExistsError(error: unknown): boolean {
 }
 
 function sessionKey(browserPageId: string, transferId: string): string {
-  return `${browserPageId}${transferId}`
+  return `${browserPageId}\u0000${transferId}`
 }

src/wsl-guest/dispatcher.test.ts

@@ -802,7 +802,7 @@ describe('RelayDispatcher', () => {
     }
 
     // Multi-byte UTF-8, astral pairs, lone surrogates, controls, quotes, and backslashes.
-    const alphabet = 'aZ9 "\\\n\r\té߀中𝄞😀𐀀�'
+    const alphabet = 'aZ9 "\\\n\r\t\u0000é߀中𝄞😀𐀀�'
 
     it('matches the pre-optimization sizing loop across randomized inputs', () => {
       const random = mulberry32(0xc0ffee)