Fix context pruner to more accurately count tokens including all fields #5282

Job	Run time
build-and-check	2m 43s
test-cli	1m 10s
test-.agents	38s
test-packages/agent-runtime	54s
test-packages/billing	40s
test-common	38s
test-packages/internal	46s
test-sdk	46s
test-integration-.agents	41s
test-web	44s
test-integration-packages/agent-runtime	43s
test-integration-common	41s
test-integration-cli	43s
test-integration-packages/internal	42s
test-integration-packages/billing	39s
test-integration-sdk	43s
test-integration-web	40s
	14m 31s

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