feat(provider): add StepFun and Step Plan presets

[Mola-maker]

StepFun users currently have to choose OpenAI Compatible and enter an endpoint manually. This adds separate StepFun and StepFun Step Plan entries under Providers → Chat Completion → Add, with the standard and subscription endpoints prefilled as documented by StepFun.

Modifications

Both presets reuse openai_chat_completion, including editable API keys/base URLs, timeout, proxy, custom headers, model discovery, manual model selection, and per-model capabilities/context/custom request parameters. Each has a distinct source ID and a theme-aware StepFun icon. English and Chinese setup instructions include the actual selector screenshot.

• This is NOT a breaking change.

Screenshots or Test Results

Visual evidence

In the existing dashboard, open Providers → Chat Completion → Add and search for StepFun. The base revision has no StepFun preset; this branch shows the two entries below. Select either entry to inspect its endpoint and advanced settings, save it with your API key, then fetch models or add a supported model ID manually.

Validation

Local validation on Windows with Python 3.12: the existing full pytest suite passed (3488 passed, 82 skipped); the new four-case preset regression test also passed. The focused OpenAI/provider run passed with 114 passed and 1 skipped. The new tests exercise the real OpenAI SDK against an in-memory HTTP transport, checking model-list and streaming/non-streaming chat URLs, authentication, custom headers, timeout, and request parameters. All four fail against the base configuration because the presets are absent.

Ruff formatting/lint, dashboard typecheck/production build, and the bilingual documentation build passed. Browser checks covered the selector, endpoint fields, advanced settings, and manual model configuration. No live StepFun inference or account-specific model availability was tested; the screenshot is from a local instance without a real API key.

Checklist

• New features have been discussed with the authors. This draft proposes the presets for review; there is no prior agreement to claim. The existing [Feature]可否支持跃阶星辰的语音转文本模型stepaudio-2.5-asr，上线了step plan，目前astrbot无法使用 #7774 and feat(provider): add StepFun StepAudio ASR provider #8362 concern StepFun ASR and are outside this change.
• Changes have been tested, with verification steps and a screenshot above.
• Affected English and Chinese WebUI instructions and screenshots have been updated. Existing navigation is unchanged.
• No new dependencies.
• No malicious code.

Summary by Sourcery

Add ready-to-use StepFun and Step Plan provider presets for OpenAI-compatible chat completion APIs.

New Features:

• Add separate StepFun and StepFun Step Plan chat-completion provider presets with preconfigured, editable API endpoints.
• Provide StepFun-specific provider icons in the dashboard.

Documentation:

• Document StepFun and Step Plan setup, endpoints, model configuration, and optional provider settings in English and Chinese guides.

Tests:

• Add regression coverage for StepFun model discovery and streaming/non-streaming chat requests, including authentication, headers, timeouts, and custom parameters.

[Soulter]

No need to add this document.

[Mola-maker]

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