Fix Secret serialization schema, applicable for unions

[sydney-runkle]

Fix #9231

Selected Reviewer: @hramezani

[sydney-runkle]

Please review

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[codspeed-hq]

CodSpeed Performance Report

Merging #9240 will not alter performance

_{Comparing fix-secret-ser (6673dc5) with main (6aab43e)}

Summary

✅ 13 untouched benchmarks

[davidhewitt]

The fix here seems a bit magical to me, can you give a bit more detail on why the original schema was wrong? I wonder if there's a deeper bug or otherwise awkward behaviour...

[sydney-runkle]

@davidhewitt,

I think there's a deeper bug, that the serialization attached to the individual python and json schemas wasn't working - definitely something that we could fix + use to clean up some schemas or at least make them a bit more intuitive.

Will report this in core.

[sydney-runkle]

Opened an issue here to address that bug in core, but this should work for now. Here's that issue: pydantic/pydantic-core#1270