chore: add byte-level debug logging for startRun parse failure (#718)

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

Author: FL4TLiN3

e2e/perstack-cli/providers.test.ts

@@ -33,40 +33,44 @@ describe.concurrent("LLM Providers", () => {
       const result = withEventParsing(cmdResult)
       const seqResult = assertEventSequenceContains(result.events, ["startRun", "completeRun"])
       if (!seqResult.passed) {
+        const lines = result.stdout.split("\n")
+        const startRunLineIdx = lines.findIndex((l) => l.includes('"startRun"'))
         process.stderr.write(
           `[DEBUG ${provider}] exitCode=${result.exitCode} eventTypes=${JSON.stringify(result.events.map((e) => e.type))}\n`,
         )
-        const startRunIdx = result.stdout.indexOf('"startRun"')
-        process.stderr.write(`[DEBUG ${provider}] startRun at index=${startRunIdx}\n`)
-        if (startRunIdx >= 0) {
-          const lineStart = result.stdout.lastIndexOf("\n", startRunIdx)
-          const lineEnd = result.stdout.indexOf("\n", startRunIdx)
-          const startRunLine = result.stdout.slice(
-            Math.max(0, lineStart),
-            lineEnd > 0 ? Math.min(lineEnd + 200, result.stdout.length) : startRunIdx + 500,
+        if (startRunLineIdx >= 0) {
+          const line = lines[startRunLineIdx]
+          const first50 = Array.from(line.slice(0, 50)).map((c) => c.charCodeAt(0))
+          const last50 = Array.from(line.slice(-50)).map((c) => c.charCodeAt(0))
+          process.stderr.write(
+            `[DEBUG ${provider}] startRunLine idx=${startRunLineIdx} len=${line.length}\n`,
           )
+          process.stderr.write(`[DEBUG ${provider}] first50codes=${JSON.stringify(first50)}\n`)
+          process.stderr.write(`[DEBUG ${provider}] last50codes=${JSON.stringify(last50)}\n`)
           process.stderr.write(
-            `[DEBUG ${provider}] startRun line (len=${startRunLine.length}): ${startRunLine.slice(0, 300)}\n`,
+            `[DEBUG ${provider}] hasNull=${line.includes("\0")} hasCR=${line.includes("\r")}\n`,
           )
           try {
-            const parsed = JSON.parse(startRunLine.trim())
-            process.stderr.write(`[DEBUG ${provider}] startRun parsed OK, type=${parsed.type}\n`)
+            JSON.parse(line)
+            process.stderr.write(`[DEBUG ${provider}] parse OK\n`)
           } catch (e) {
-            process.stderr.write(
-              `[DEBUG ${provider}] startRun PARSE FAIL: ${(e as Error).message}\n`,
-            )
-            process.stderr.write(
-              `[DEBUG ${provider}] startRun last200: ...${startRunLine.slice(-200)}\n`,
-            )
+            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`,
+              )
+            }
           }
+        } else {
+          process.stderr.write(`[DEBUG ${provider}] startRun NOT FOUND in any line\n`)
         }
-        const lines = result.stdout.split("\n")
-        process.stderr.write(
-          `[DEBUG ${provider}] totalLines=${lines.length} lineStarts=${lines
-            .slice(0, 10)
-            .map((l) => l.slice(0, 30))
-            .join(" | ")}\n`,
-        )
       }
       expect(result.exitCode).toBe(0)
       expect(seqResult.passed).toBe(true)