fix: encode ActivityOptions.summary as json/plain payload

[thejens]

What was changed

ActivityOptions::into_command now encodes the summary field using as_json_payload() (json/plain encoding) instead of .into() (binary/plain encoding).

Why?

LocalActivityOptions::into_command (same file, line ~203) already encodes its summary via as_json_payload(). The ActivityOptions path used .into() which produces a binary/plain payload — the Temporal UI cannot decode this and shows "Decoding failed" instead of the summary text.

Checklist

1. Closes — no existing issue

2. How was this tested: built and ran locally, see screenshots:

Before:

After:

3. Any docs updates needed? No

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[Sushisource]

Thanks! We actually need to get the user's data converter all the way in here... as_json_payload should actually go away eventually.

So probably we'll just need to do that properly before the next release. If you're interested in updating the PR to try to do it that way, please do!