fix: correctly create multipart requests for is activities #2811

Job	Run time
discover-testcases	4s
apicalls-testcase / staging	56s
apicalls-testcase / cloud	46s
eval-spans-testcase / alpha	1m 43s
eval-spans-testcase / cloud	1m 57s
eval-input-overrides / staging	33s
eval-telemetry-testcase / cloud	2m 20s
calculator-crash-evals / alpha	1m 24s
eval-spans-testcase / staging	1m 51s
basic-testcase / staging	36s
apicalls-testcase / alpha	56s
calculator-evals / staging	3m 1s
eval-input-overrides / alpha	35s
calculator-evals / alpha	2m 42s
calculator-crash-evals / staging	1m 57s
langchain-cross / alpha	42s
basic-testcase / cloud	31s
calculator-evals / cloud	2m 52s
basic-testcase / alpha	34s
eval-input-overrides / cloud	29s
tools-evals / staging	2m 5s
calculator-crash-evals / cloud	1m 50s
eval-telemetry-testcase / alpha	2m 16s
eval-telemetry-testcase / staging	2m 26s
tools-evals / alpha	2m 0s
langchain-cross / staging	48s
tools-evals / cloud	2m 2s
langchain-cross / cloud	45s
summarize-results	3s
	40m 44s

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