fix: correctly create multipart requests for is activities #2809

Job	Run time
discover-testcases	6s
eval-input-overrides / alpha	29s
calculator-crash-evals / cloud	2m 0s
calculator-crash-evals / alpha	1m 31s
basic-testcase / cloud	32s
langchain-cross / cloud	42s
eval-telemetry-testcase / cloud	2m 19s
calculator-evals / staging	2m 55s
langchain-cross / staging	42s
calculator-crash-evals / staging	1m 49s
eval-telemetry-testcase / staging	2m 16s
tools-evals / cloud	2m 10s
basic-testcase / staging	34s
calculator-evals / cloud	2m 59s
basic-testcase / alpha	29s
apicalls-testcase / cloud	52s
calculator-evals / alpha	2m 43s
eval-input-overrides / cloud	29s
apicalls-testcase / alpha	49s
eval-spans-testcase / alpha	1m 40s
apicalls-testcase / staging	51s
tools-evals / alpha	2m 1s
eval-input-overrides / staging	32s
eval-spans-testcase / cloud	1m 50s
eval-spans-testcase / staging	1m 49s
eval-telemetry-testcase / alpha	2m 13s
langchain-cross / alpha	46s
tools-evals / staging	2m 10s
summarize-results	3s
	40m 21s

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