[API] Add synchronous experiment API with Path inputs

[podkidyshev]

Summary

• Add synchronous run_experiment, validate_scenario, list_experiments, and get_experiment in cloudai.api. Scenario and system inputs are configuration-file Path objects; get_experiment accepts a directory or JSON file path.
• Share execution logic with the CLI. Allow parsing errors to propagate to API callers, raise job-submission errors, and select single-sbatch runners per invocation without changing the registry.
• Return the existing experiment model, allocate unique result directories, and document the API. Configuration parsing otherwise uses the existing parser.

Test Plan

• Automated CI.
• Manual macOS standalone test: validated a scenario file referencing sleep.toml relatively, ran Sleep synchronously, and verified the completed experiment through both listing and retrieval.

PASS: Path inputs, relative test reference, synchronous Sleep, listing and retrieval

Additional Notes

Based on #1064. Follow-ups: #1061 adds TOML-text inputs; #1062 adds background execution.

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