ci(metrics): collapse Average Code Size Metrics table when no base

When no base metrics are available to compare against, the PR comment
falls back to the combined "TinyUSB Average Code Size Metrics" report
(build.yml copies metrics.md to metrics_compare.md). That posted the
full per-example size table inline, cluttering the comment.

Wrap the table in a <details><summary>Size table</summary> block so the
heading stays visible but the detail is collapsed by default, matching
the Size Difference Report's collapsible sections.

Co-Authored-By: Claude Fable 5 <[email protected]>

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