Fix `Secret` serialization schema, applicable for unions (#9240) #8

Job	Run time
lint 3.8	3m 26s
docs-build	1m 21s
lint 3.9	3m 25s
lint 3.10	3m 17s
lint 3.11	1m 13s
lint 3.12	3m 41s
test ubuntu / 3.8	2m 0s
test ubuntu / 3.9	1m 55s
test ubuntu / 3.10	2m 3s
test fastapi	1m 58s
test ubuntu / 3.11	1m 59s
test ubuntu / 3.12	2m 25s
test memray	4m 0s
test macos / 3.8	7m 34s
test macos / 3.9	6m 19s
mypy 1.0.1 / 3.10	4m 31s
test macos / 3.10	4m 45s
mypy lockfile / 3.10	4m 2s
test macos / 3.11	7m 41s
mypy 1.2.0 / 3.10	4m 17s
test macos / 3.12	7m 36s
mypy 1.4.1 / 3.10	2m 48s
mypy 1.5.1 / 3.10	2m 42s
test windows / 3.8	4m 32s
mypy lockfile / 3.8	3m 28s
test windows / 3.9	4m 34s
mypy lockfile / 3.9	3m 48s
test windows / 3.10	4m 44s
mypy lockfile / 3.11	3m 50s
test windows / 3.11	5m 2s
mypy 1.5.1 / 3.12	3m 16s
test windows / 3.12	6m 18s
test ubuntu / pypy3.9	9m 36s
test ubuntu / pypy3.10	9m 37s
test pydantic plugin	1m 18s
test typing-extensions 4.6.1 on Python 3.8	2m 12s
test typing-extensions 4.6.1 on Python 3.9	2m 14s
test typing-extensions 4.6.1 on Python 3.10	2m 4s
test typing-extensions 4.6.1 on Python 3.11	2m 8s
test typing-extensions 4.6.1 on Python 3.12	2m 20s
test typing-extensions main on Python 3.8	2m 13s
test typing-extensions main on Python 3.9	2m 14s
test typing-extensions main on Python 3.10	2m 6s
test typing-extensions main on Python 3.11	2m 19s
test typing-extensions main on Python 3.12	2m 27s
check	4s
coverage-combine	22s
release	0s
Send tweet	0s
	2h 47m 44s

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