refactor(cabal-install, cabal-install-solver): avoid using "error" in validateSolverResult

[andreabedini]

• Add a step helper and a MonadFail instance to Progress
• Refactor validateSolverResult to integrate into the Progress monad chain
  using fail instead of error, giving structured failure instead of an
  uncatchable exception

[Copilot]

Pull request overview

This PR refactors the dependency-solver result validation path to avoid throwing uncatchable exceptions (error) by integrating validation into the Progress monad chain, enabling structured failure (Fail) instead.

Changes:

• Add a step helper and a MonadFail instance for Progress (String failures).
• Refactor resolveDependencies/validateSolverResult to use do-notation and fail instead of error.
• Adjust unit tests to avoid name collisions with the new step helper.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
cabal-install/tests/UnitTests/Distribution/Solver/Modular/RetryLog.hs	Renames test lambda variables to avoid clashing with newly introduced step.
cabal-install/src/Distribution/Client/Dependency.hs	Converts validation from error to Progress failure and threads it through the Progress chain.
cabal-install-solver/src/Distribution/Solver/Types/Progress.hs	Adds step helper and MonadFail instance to support structured failure in Progress.

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