feat: add Synthorai LLM requester

[cuihuan]

Adds Synthorai (https://synthorai.io) as a supported LLM requester, following the same manifest-only pattern as the TokenLab requester (#2326), merged 2026-07-15 — same two files, an .svg and a <name>chatcmpl.yaml.

Synthorai is an OpenAI/Anthropic-compatible LLM gateway routing to 113 models across 11 upstream providers (Claude, GPT, Gemini, GLM, Kimi, DeepSeek, Qwen, etc.) at direct upstream pricing, no markup. Docs: https://synthorai.io/docs

Changes

• src/langbot/pkg/provider/modelmgr/requesters/synthoraichatcmpl.yaml
• src/langbot/pkg/provider/modelmgr/requesters/synthorai.svg

Points execution.python at the existing generic ./chatcmpl.py / OpenAIChatCompletions (same executor the base OpenAI entry uses) rather than a new per-provider .py file, since this is a plain OpenAI-compatible passthrough with no custom request/response shaping needed.

[github-actions]

All contributors have signed the CLA. ✅ 所有贡献者均已签署 CLA。
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[cuihuan]

I have read the CLA Document and I hereby sign the CLA

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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[cuihuan]

Correcting the precedent I cited, because I cited the wrong one and it happens to be the worst possible wrong one.

The description said this follows "the same manifest-only pattern as the OrcaRouter integration (#2411)". #2411 was closed without merging on 2026-08-13 — so I was holding up a rejected PR as evidence that this kind of contribution gets accepted. I didn't check its state before citing it.

The correct precedent is #2326 (feat(provider): add TokenLab requester), merged 2026-07-15 by an outside contributor — and it is exactly the same two files this PR touches: a .svg and a <name>chatcmpl.yaml under requesters/. Description updated.

Also restating the disclosure where a reviewer will actually see it: I'm affiliated with Synthorai, so this is a vendor adding its own requester rather than a user request, and I'm not claiming demand for it.

CLA is signed. Happy to change anything about the manifest if the shape isn't what you want.