Tolerate OpenAI model schema drift in the agents payload converter (#… #5369

Job	Run time
test-latest-deps	9m 45s
check-protos	7m 34s
build-lint-test (3.14, ubuntu-latest)	18m 39s
build-lint-test (3.14, macos-arm)	7m 58s
cloud-test	1m 44s
build-lint-test (3.10, macos-arm)	9m 2s
build-lint-test (3.10, ubuntu-latest)	13m 52s
build-lint-test (3.14, ubuntu-arm)	11m 8s
build-lint-test (3.14, windows-latest)	27m 59s
build-lint-test (3.10, ubuntu-arm)	10m 49s
build-lint-test (3.10, windows-latest)	20m 33s
features-tests / test	8m 5s
	2h 27m 8s

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