Fix TypeScript error: only include cost in providerMetadata when defined #5590

Job	Run time
build-and-check	2m 53s
test-integration-packages/internal	1m 7s
test-integration-packages/billing	1m 9s
test-integration-sdk	49s
test-integration-packages/agent-runtime	47s
test-integration-web	44s
test-integration-cli	41s
test-integration-common	46s
test-common	38s
test-packages/agent-runtime	1m 1s
test-cli	2m 31s
test-packages/billing	39s
test-web	51s
test-.agents	42s
test-packages/internal	49s
test-sdk	45s
	16m 52s

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