Fix remaining object serialization callsites

[Ankcorn]

Summary

Follow-up to #6662.

This fixes the remaining object-construction paths that were still using js.obj(keys, values) producing unserializable objects.

Tests

regression tests cover fetch and queue but not email because I couldn't find an existing e2e test harness for email

[ask-bonk]

Good - JsObject (via JsBase) has operator JsValue() so returning a JsObject from a function that returns jsg::JsValue is fine.

Everything looks clean. This is a straightforward, correct follow-up fix. No actionable issues.

LGTM

github run

[fhanau]

Thank you for doing this – I said I'd investigate the underlying issue, haven't gotten to that yet.
For the tests being added here, can we all put them in the same test file? These and the existing test at src/workerd/api/tests/stw-jsrpc-dataclone-test.wd-test are all about the same underlying issue, consolidating this instead of having separate files for each test will make maintenance easier.

[codspeed-hq]

Merging this PR will improve performance by 40.28%

⚡ 1 improved benchmark
✅ 71 untouched benchmarks
⏩ 129 skipped benchmarks¹

Performance Changes

	Benchmark	BASE	HEAD	Efficiency
⚡	simpleStringBody[Response]	27.2 µs	19.4 µs	+40.28%

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_{Comparing tankcorn/fix-remaining-obj-serialization (fbc2d59) with main (d571ec1)}

Footnotes

1. 129 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩

[Ankcorn]

Thank you for doing this – I said I'd investigate the underlying issue, haven't gotten to that yet. For the tests being added here, can we all put them in the same test file? These and the existing test at src/workerd/api/tests/stw-jsrpc-dataclone-test.wd-test are all about the same underlying issue, consolidating this instead of having separate files for each test will make maintenance easier.

Thanks for the suggestion, I have consolidated the tests :)