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. The diff is limited to provider presets, icon mappings, and regression tests.

• 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 and the four preset regression cases were rerun after the documentation removal and passed. Dashboard typecheck/production build and browser checks passed on the initial implementation, which is unchanged. 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 retained at the original immutable commit as review evidence, rather than added to the documentation.

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.
• No documentation changes are included, as requested in maintainer review. These presets use the existing provider configuration flow; navigation is unchanged.
• No new dependencies.
• No malicious code.

[Soulter]

No need to add this document.

[Mola-maker]

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