[Refactor][Config] Remove MLAPO environment variable

[kk-ss1999]

What this PR does / why we need it?

Removes the legacy VLLM_ASCEND_ENABLE_MLAPO environment variable after its behavior was migrated to additional_config.enable_mlapo. Runtime code, deployment examples, test configurations, and documentation are migrated to the config field where applicable.

This is one independently reviewable part of the seven-variable split of #14637.

Does this PR introduce any user-facing change?

Yes. VLLM_ASCEND_ENABLE_MLAPO is no longer read. Users must configure the behavior with --additional-config '{"enable_mlapo":true}'.

How was this patch tested?

• ruff check and ruff format --check on changed Python files

• Python byte-compilation of changed Python files

• YAML parsing of changed YAML files

• JSON parsing of concrete additional-config payloads

• Scan confirming no active references to VLLM_ASCEND_ENABLE_MLAPO remain outside migration documentation and the ignored-variable unit test

• NPU end-to-end tests were not run in this Windows environment

• vLLM version: v0.27.1

• vLLM main: vllm-project/vllm@58d3918

[gemini-code-assist]

Summary of Changes

Hello, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request completes the migration of the MLAPO configuration from an environment variable to the additional_config JSON structure. This change is part of a broader effort to consolidate configuration options, providing a more structured and consistent approach for managing Ascend-specific features across the codebase and deployment environments.

Highlights

• Environment Variable Removal: Removed the legacy VLLM_ASCEND_ENABLE_MLAPO environment variable to standardize configuration management.
• Configuration Migration: Migrated the MLAPO setting to the additional_config.enable_mlapo JSON field, requiring users to update their deployment configurations.
• Documentation and Test Updates: Updated all relevant documentation, end-to-end test configurations, and unit tests to reflect the new configuration method.

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[github-actions]

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[gemini-code-assist]

Code Review

This pull request removes the legacy VLLM_ASCEND_ENABLE_MLAPO environment variable and migrates its functionality to the additional_config.enable_mlapo configuration field across the codebase, including documentation and test configurations. The reviewer suggests adding robust error handling to the _extract_features function in tests/e2e/nightly/multi_node/external_dp/scripts/utils.py to safely handle potential parsing errors when processing the additional configuration.

[gemini-code-assist]

The command parsing logic in _extract_features lacks error handling when parsing --additional-config or --additional_config. If the command list ends with the flag (causing an IndexError) or if the JSON string is malformed (causing a json.JSONDecodeError), the test runner will crash. Wrapping this block in a try-except block ensures robustness.

Suggested PR Title:

[Ops][Misc] Remove legacy MLAPO environment variable and migrate to additional_config

Suggested PR Summary:

### What this PR does / why we need it?

This PR removes the legacy `VLLM_ASCEND_ENABLE_MLAPO` environment variable and migrates its behavior to `additional_config.enable_mlapo`. This ensures a cleaner configuration interface and aligns with the ongoing effort to consolidate environment variables into the configuration fields.

### Does this PR introduce _any_ user-facing change?

Yes. The `VLLM_ASCEND_ENABLE_MLAPO` environment variable is no longer supported. Users must now configure this behavior using `--additional-config '{"enable_mlapo": true}'`.

### How was this patch tested?

- Verified configuration migration and fallback behavior in unit tests (`tests/ut/test_ascend_config.py`).
- Updated end-to-end test configurations to use the new `additional_config` option.

        try:
            flag_index = command.cmd.index(flag)
            additional_config = json.loads(command.cmd[flag_index + 1])
            if additional_config.get("enable_mlapo") and "mlapo" not in features:
                features.append("mlapo")
        except (IndexError, json.JSONDecodeError):
            pass

[github-actions]

This pull request has conflicts, please resolve those before we can evaluate the pull request.