feat(solver): surface SolverProblemsException via two-layer Result

solve() previously dropped the SolverProblemsException and returned a
placeholder anyhow error, because its Rc<RefCell<Rule>> payload is not
Send+Sync and cannot ride anyhow::Error. Return
anyhow::Result<Result<LockTransaction, SolverProblemsException>> instead,
keeping fatal errors on the outer Result and the recoverable exception on
the inner one.

This lets the three Installer callsites port their PHP catch handlers
faithfully (pretty-string output, GithubActionError emit, getCode-based
exit codes) and lets solver_test.rs assert getProblems/getCode/
getPrettyString. The four ported assertion tests stay ignored: their
pretty-string path still reaches unimplemented todo!()s downstream.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

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