fix(cli): fail loudly on unrecognized --solvers names

[HarshRajSinghania]

Fixes #178.

Summary

--solvers names that do not match a registered solver now abort the CLI with the unknown name, a close-match suggestion when one exists, and the list of available solver names.

Motivation

#178 reports that --solvers=fenics printed unknown solver(s): fenics — skipping and then no matching solvers after filtering — running all. The registered name is fenics_structural. Running every solver after a typo is the wrong fallback.

Maintainer request on the issue: print available solvers, and treat this as a loud failure rather than a silent run-all.

Implementation

• Added _fail_unknown_solvers() so the unknown-name path is shared.
• Flat --solvers CSV still uses _validate_solver_csv() and now calls that helper (same red error + available list + typer.Exit(1)).
• Per-problem maps (problem=name) no longer warn-and-skip on an unknown name for that problem; they fail the same way against that problem's solver list.
• A problem with a flat CSV that simply does not include any of this problem's solvers still skips that problem only. That is unchanged and is not a typo.

Testing

Commands actually run:

• python3 -m py_compile mosaic/benchmarks/cli/_helpers.py tests/test_cli_helpers.py — passed
• AST parse of both files — passed

The full mosaic package import requires jax and the rest of the solver stack, which is not available in this environment, so pytest tests/test_cli_helpers.py was not executed here. New tests added:

• per-problem unknown name raises SystemExit
• _fail_unknown_solvers prints available names and a suggestion
• _validate_solver_csv('fenics') exits when only fenics_structural is registered
• _validate_solver_csv('fenics_structural') succeeds

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[dionhaefner]

@HarshRajSinghania Thanks for the contribution! Please make sure to commit using one of your registered e-mail addresses, so our CLA bot can pick up your identity. Happy to give this a review as soon as the CLA is done :)