[FEAT] Enhanced Testing

[bbengfort]

Scope of changes

Enhances testing of the confire package.

Type of change

• new feature
• bug fix
• documentation
• testing
• technical debt
• other (describe)

Author checklist

• I have manually tested the change and/or added automation in the form of unit tests or integration tests
• I have added new test fixtures as needed to support added tests

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Note

Medium Risk
Medium risk because it changes validation gathering/flattening (including struct-level Validate() support) and adds new public error-introspection helpers that could affect error types/order observed by callers.

Overview
Adds a new contest.Env test helper to set/clear scoped environment variables with automatic restoration, along with new assert helpers (NotOk plus environment assertions) and README documentation for using them.

Expands validation behavior by allowing the root spec to implement validate.Validator, and by flattening/normalizing nested errors.ValidationErrors into a consistent list; the top-level confire package now exposes ValidationErrors() and IsValidationErrors() helpers to inspect those errors. Includes new end-to-end tests (confire_test.go) covering env loading, defaults, and multi-error validation output ordering.

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