Fix NaN handling in ttftBucketIndex

The function didn't validate that ttftMs is a finite number. If ttftMs was
NaN or Infinity, Math.max(NaN, 1) would return NaN, causing Math.log(NaN)
to return NaN, and the entire calculation would produce NaN.

Added Number.isFinite() check to default to 0 for invalid numbers.

The existing test suite (11 tests, 995 assertions) already covers edge cases
including NaN, Infinity, and negative values, and all pass with this fix.

Author: pavankumar-vh

common/src/util/ttft-histogram.ts

@@ -37,7 +37,8 @@ const LN_BASE = Math.log(TTFT_HISTOGRAM_BASE)
  * Sub-millisecond and zero samples land in bucket 0 rather than at -Infinity.
  */
 export function ttftBucketIndex(ttftMs: number): number {
-  const index = Math.floor(Math.log(Math.max(ttftMs, 1)) / LN_BASE)
+  const safeTtftMs = Number.isFinite(ttftMs) ? ttftMs : 0
+  const index = Math.floor(Math.log(Math.max(safeTtftMs, 1)) / LN_BASE)
   return Math.min(TTFT_HISTOGRAM_BUCKET_COUNT - 1, Math.max(0, index))
 }