Fix SGLANG_GRPC_PORT overflow for high --port values

[ajinkyajawale14499]

Motivation

Closes #29416. When --port is greater than 55535, launching the server crashes with ValueError: SGLANG_GRPC_PORT (...) must be between 1 and 65535, even though the user never set SGLANG_GRPC_PORT. The reported command used --port 65438.

The cause is in ServerArgs: when SGLANG_GRPC_PORT is unset, grpc_port is derived as port + 10000, and the following range check then rejects anything above 65535.

Modifications

• python/sglang/srt/server_args.py: when deriving grpc_port from --port, keep port + 10000, but fall back to port - 10000 when that would exceed 65535. port + 10000 > 65535 implies port > 55535, so port - 10000 > 45535, which stays in range and still differs from --port. An explicitly set SGLANG_GRPC_PORT is left untouched and still validated by the existing range check.
• test/registered/unit/server_args/test_server_args.py: add TestGrpcPortDerivation covering the high-port case (no longer raises, stays in range, differs from --port) and the normal case (port + 10000).

Accuracy Tests

N/A. This is an argument-parsing change and does not affect model output.

Speed Tests and Profiling

N/A.

Checklist

• Format your code according to the pre-commit guide (isort / ruff / black / codespell pass).
• Add unit tests (CPU, base-a-test-cpu).
• Update documentation. N/A.
• Provide accuracy and speed benchmark results. N/A.
• Follow the SGLang code style guidance.

The derivation was verified locally across several --port values; the ServerArgs construction test runs in CPU CI (developed on Apple Silicon, so I couldn't run the GPU suite locally).

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CI States

Latest PR Test (Base): ❌ Run #28294831452
Latest PR Test (Extra): ❌ Run #28294831374

[gemini-code-assist]

Code Review

This pull request updates the gRPC port derivation logic to prevent port overflow when a high port is configured. If the derived port (port + 10000) exceeds 65535, it falls back to port - 10000. Unit tests have been added to verify this behavior. The reviewer suggested simplifying the condition self.port + 10000 <= 65535 to self.port <= 55535 for better readability.

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