fix: use actual newline in E2E event parser buffer concatenation

[FL4TLiN3]

Summary

• Fixed parseEvents using "\\n" (literal backslash-n, 2 chars) instead of "\n" (actual newline) when joining split lines in the buffer
• This caused JSON.parse to fail on large events (like startRun with full instruction text) that were split across multiple stdout lines
• The startRun event JSON grew after splitting InstructionMessage into multiple TextParts, making it more likely to exceed stdout buffer boundaries and get split
• Also removes temporary debug logging from providers.test.ts and continue.test.ts

Root cause

When a large JSON event line gets split across multiple lines in stdout, the parser buffers it. But buffer += "\\n" + line inserted literal \n characters (not actual newlines), producing invalid JSON like ...text\nmore text... where the \n is two characters instead of a proper JSON escape sequence.

Test plan

• All unit tests pass
• Typecheck passes
• E2E tests pass in CI (this was the root cause of consistent OpenAI and continue test failures)

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