Improve Invalid Configuration Errors

[bbengfort]

Scope of changes

Improves the invalid config errors and exports some standard handling for users to define errrors.

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
Renames and reshapes the public validation-error surface (replacing ValidationError/ErrMissingRequiredField with InvalidConfig/ErrMissingRequired) and adds new exported helpers, which can be breaking for downstream callers and changes error-type matching behavior.

Overview
Improves configuration validation error reporting by introducing a new InvalidConfig error type (with helpers Required, Invalid, Parse, and Wrap) and switching validation to emit these field-scoped errors.

Reworks multi-error handling via ValidationErrors and a custom Join that flattens/returns InvalidConfig-based collections while falling back to errors.Join when non-validation errors are mixed in.

Updates the confire convenience API to expose InvalidConfig extraction/checks and re-exports the new error constructors, and adjusts tests accordingly (including moving ParseError into its own file and test).

^{Written by Cursor Bugbot for commit 2b0062a. Configure here.}