[cli] fix: detect model-path from --config in sglang serve

[clayrosenthal]

Motivation

Fixes #36105

Running sglang serve --config my_config.yaml fails with Error: --model-path is required even when the YAML config contains a model-path key, while python -m sglang.launch_server --config my_config.yaml works fine. This is because try_get_model_path() in python/sglang/cli/utils.py only scanned the raw CLI argv for --model-path/--model and ignored values supplied through --config, so backend auto-detection was skipped and get_model_path() raised.

Modifications

• Added _try_get_model_path_from_config() in python/sglang/cli/utils.py: a best-effort helper that reads the --config <file> YAML and returns the value of a model-path / model_path / model key. It mirrors ConfigArgumentMerger semantics (space-form --config only, key normalization, last-in-file wins) and never raises — missing/unreadable/malformed files, non-dict roots, and non-string keys or empty/non-string values all return None, so the authoritative parser in prepare_server_args remains the source of truth.
• try_get_model_path() now falls back to this helper only when no explicit --model-path/--model CLI flag is present, preserving CLI > config > defaults precedence.
• Added import yaml (PyYAML is already a hard dependency).

Accuracy Tests

• New test/registered/unit/cli/test_cli_utils.py (17 tests, registered in base-a-test-cpu): config-key detection (model-path/model_path/model), CLI-flag precedence, nonexistent/empty/malformed/non-dict configs, non-string keys, falsy/non-string values, last-key-wins, and CLI flag sanity (--model-path X, --model X, --model=X). All pass.
• Existing test/registered/unit/cli/test_serve_backends.py (12 tests) still passes, including test_explicit_backend_can_support_config_only_requests (missing pipeline.yaml → model_path is None, never raises).

Speed Tests and Profiling

N/A — pure CLI argument-parsing helper, best-effort, not on any serving hot path.

Checklist

• Format your code according to the Format code with pre-commit. (ruff/black/isort verified clean)
• Add unit tests according to the Run and add unit tests. (17 new tests in test/registered/unit/cli/test_cli_utils.py)
• Update documentation according to Write documentations. (N/A — no user-facing docs affected)
• Provide accuracy and speed benchmark results according to Test the accuracy and Benchmark the speed. (N/A — not on any serving hot path)
• Follow the SGLang code style guidance.

Review and Merge Process

1. Ping Merge Oncalls to start the process. See the PR Merge Process.
2. Get approvals from CODEOWNERS and other reviewers.
3. Trigger CI tests with comments or contact authorized users to do so.
   • Common commands include /tag-and-rerun-ci, /tag-run-ci-label, /rerun-failed-ci
4. After green CI and required approvals, ask Merge Oncalls or people with Write permission to merge the PR.

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CI States

Latest PR Test (Base): ❌ Run #32662327838
Latest PR Test (Extra): ❌ Run #32662327718
Latest PR Test (AMD ROCm 7.2): ❌ Run #32662327869

[clayrosenthal]

@slin1237 Could you take a look at this? Thank you