Remove remaining chunk terminology from built-in-ai metrics and streaming UI

[Copilot]

The built-in AI demo still exposed chunk terminology in metric labels, DOM IDs, CSS selectors, and streaming code paths after the initial text replacement. This change normalizes the terminology to token across source, templates, and generated playground output so chunk no longer appears anywhere in built-in-ai.

• Metrics + UI terminology

  • Renames user-facing labels such as First chunk latency and chunk/sec to First token latency and token/sec.
  • Updates tracked generated playground pages to keep checked-in output aligned with the source templates.

• DOM + styling consistency

  • Renames metric element IDs from first-chunk-latency-metric, chunks-metric, and chunk-rate-metric to first-token-latency-metric, tokens-metric, and token-rate-metric.
  • Updates CSS selectors that depend on those IDs.

• Streaming code cleanup

  • Renames local streaming variables, counters, and metric helpers from chunk-based naming to token-based naming.
  • Applies the same cleanup in sample scripts so the term is removed from logs and stream handling code as well.

<!-- before -->
<span class="metric">First chunk latency: <span class="value" id="first-chunk-latency-metric"></span> ms</span>
<span class="metric">Rate: <span class="value" id="chunk-rate-metric"></span> chunk/sec</span>

<!-- after -->
<span class="metric">First token latency: <span class="value" id="first-token-latency-metric"></span> ms</span>
<span class="metric">Rate: <span class="value" id="token-rate-metric"></span> token/sec</span>