chore: scan for non-ASCII chars and binary search parse failure region (#719)

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

Author: FL4TLiN3

e2e/perstack-cli/providers.test.ts

@@ -50,23 +50,45 @@ describe.concurrent("LLM Providers", () => {
           process.stderr.write(
             `[DEBUG ${provider}] hasNull=${line.includes("\0")} hasCR=${line.includes("\r")}\n`,
           )
+          const badChars: { i: number; code: number }[] = []
+          for (let i = 0; i < line.length; i++) {
+            const code = line.charCodeAt(i)
+            if (code < 32 || code > 126) {
+              badChars.push({ i, code })
+            }
+          }
+          if (badChars.length > 0) {
+            process.stderr.write(
+              `[DEBUG ${provider}] nonAscii=${JSON.stringify(badChars.slice(0, 20))}\n`,
+            )
+          } else {
+            process.stderr.write(`[DEBUG ${provider}] allPrintableAscii=true\n`)
+          }
           try {
             JSON.parse(line)
             process.stderr.write(`[DEBUG ${provider}] parse OK\n`)
           } catch (e) {
             const msg = (e as Error).message
             process.stderr.write(`[DEBUG ${provider}] parse FAIL: ${msg}\n`)
-            const posMatch = msg.match(/position (\d+)/)
-            if (posMatch) {
-              const pos = Number.parseInt(posMatch[1])
-              const around = Array.from(line.slice(Math.max(0, pos - 5), pos + 5)).map((c) =>
-                c.charCodeAt(0),
-              )
-              process.stderr.write(`[DEBUG ${provider}] chars@${pos}: ${JSON.stringify(around)}\n`)
-              process.stderr.write(
-                `[DEBUG ${provider}] text@${pos}: ${JSON.stringify(line.slice(Math.max(0, pos - 20), pos + 20))}\n`,
-              )
+            let lo = 0
+            let hi = line.length
+            while (hi - lo > 100) {
+              const mid = Math.floor((lo + hi) / 2)
+              try {
+                JSON.parse(line.slice(0, mid) + "}")
+                lo = mid
+              } catch {
+                hi = mid
+              }
             }
+            process.stderr.write(`[DEBUG ${provider}] failRegion=[${lo},${hi}]\n`)
+            const regionCodes = Array.from(line.slice(lo, Math.min(hi, lo + 100))).map((c) =>
+              c.charCodeAt(0),
+            )
+            process.stderr.write(`[DEBUG ${provider}] regionCodes=${JSON.stringify(regionCodes)}\n`)
+            process.stderr.write(
+              `[DEBUG ${provider}] regionText=${JSON.stringify(line.slice(lo, Math.min(hi, lo + 100)))}\n`,
+            )
           }
         } else {
           process.stderr.write(`[DEBUG ${provider}] startRun NOT FOUND in any line\n`)