Rjf/one or many error

[robfitzgerald]

this PR addresses a limitation in the OneOrMany type during deserialization errors. if the inner type T fails to deserialize, the OneOrMany deserialization error gobbles up the inner error message and only returns (for, in this case, a config file named config_file.toml):

failure building compass app: while reading config_file.toml: data did not match any variant of untagged enum OneOrMany

which is the default deserialization error of an #[serde(untagged)] enum. this PR implements a custom deserializer that will not fail to propagate the inner error message. tests are added that demonstrate the improvement.

[Copilot]

Pull request overview

This PR improves config deserialization diagnostics by replacing OneOrMany<T>’s derived Serde behavior with a custom Deserialize implementation that preserves inner T deserialization errors, and adds unit tests to validate the improved error messages.

Changes:

• Implement a custom serde::de::Visitor + Deserialize for OneOrMany<T> to propagate inner errors.
• Remove derived Deserialize/#[serde(untagged)] usage in favor of the custom deserializer.
• Add unit tests covering single vs many cases and error-message propagation.

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[nreinicke]

This all makes sense to me