[Rust] Fixed Rust default isAnyType causing compiler issues.

[ranger-ross]

This PR fixes an issue where types with an unknown type where not templated properly causing a compiler error. (see issue linked below)

The approach to fix this is to default unknown types for path, query and header params to String instead of serde_json::Value.
The rational is that it is quiet rare for json to be passed in non-body params.

fixes #18526

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cc: @frol @farcaller @richardwhiuk @paladinzh @jacob-pro

[wing328]

can you please review the build failure when you've time?

https://github.com/OpenAPITools/openapi-generator/actions/runs/13242785126/job/36961850037?pr=20631

you may need to manually remove the auto-generated test file and regenerate the samples again (run it twice)

[ranger-ross]

Sorry, looks like there is a test in thread_test.rs that needs to be updated manually.

Just pushed a commit that should fix CI

[wing328]

thanks for the fix, which has been merged