chore: extract record_traced_span for the span + request-store ceremony

[Nic-Polumeyv]

Form actions, both load paths, remote calls and remote form posts each hand-build the same wrapper: open a span, merge_tracing the event, re-enter the request store, run the work. This extracts it into record_traced_span next to record_span and rewrites those five sites.

The two remote-functions.js hunks overlap with #16064, which restructures that file. Happy to rebase whichever lands second.

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[Nic-Polumeyv]

Superseded by #16600, which removes these call sites instead of extracting them.