test: add unit tests for trace processing helpers

[ZayanKhan-12]

Why

src/trace-processing/parse.ts exports several pure, browser-free helpers, but tests/trace-processing/parse.test.ts only exercised the happy parse path, the trace summary snapshot, and the undefined-buffer error. This PR adds direct unit tests for the remaining exported helpers and branches, so regressions in the discriminant logic or the error contracts are caught without needing a browser.

Coverage added

• traceResultIsSuccess
  • returns true for the result of a real successful parse (fixture trace)
  • returns true for a TraceResult whose insights is null (the guard must only depend on parsedTrace)
  • returns false for the result of a failed parse
  • returns false for a TraceParseError object
  • returns false for a near-miss where the error message text is 'parsedTrace' — the guard checks the key, not values
• parseRawTraceBuffer (previously untested error branches)
  • empty buffer → 'Decoding the trace buffer returned an empty string.'
  • invalid JSON → returns a TraceParseError instead of throwing
  • valid JSON that is not a trace → returns a TraceParseError instead of throwing
• getInsightOutput (previously untested)
  • formatted output for a known insight (NAVIGATION_0 / LCPBreakdown from the web-dev-with-commit fixture)
  • error when the trace has no insights
  • error for an unknown insight set id
  • error for an unknown insight name

Tests only — no changes under src/, and no new snapshots (the insight output is asserted with a targeted match so devtools-frontend formatter updates don't churn a snapshot).

Testing

• npm run build — clean
• node scripts/test.js tests/trace-processing/parse.test.ts — 15/15 pass (3 suites)
• npm run test:no-build — full suite green (exit 0)
• npm run check-format — eslint + prettier clean

🤖 Generated with Claude Code

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