[diffusion] refactor: scope model-specific API parameters

[mickqian]

Motivation

Model-specific diffusion controls should not accumulate in the shared OpenAI request or base sampling schemas. The current API also hardcodes Cosmos3, ERNIE-Image, and Ideogram fields in generic endpoints, while documented LTX-2.5 online controls are not forwarded at all.

LongCat-Image additionally needs its missing public cookbook/navigation coverage.

Modifications

• Add the LongCat-Image deployment cookbook, navigation, README entry, and cookbook index card.
• Keep LongCat, Cosmos3, ERNIE-Image, Ideogram 4, and LTX-2.5 request controls in their model-owned sampling parameter classes.
• Extract only extensions declared by the active model and backed by fields on its sampling dataclass.
• Move Cosmos3 aliases, sound-duration derivation, and action-field normalization out of the generic video endpoint and into Cosmos3SamplingParams lowering.
• Forward the documented LTX-2.5 auto_duration and use_diffusion_decoder online controls.
• Preserve root-level JSON, OpenAI SDK extra_body, video multipart extra_params, and legacy image extra_args; root-level fields retain precedence.
• Document the extension contract in the OpenAI API guide and update the Cosmos3 and LTX-2.5 cookbooks.

Accuracy Tests

Not run locally, following the repository-local SGLang-Diffusion development guidance. This refactor does not change model computation or default sampling values.

Speed Tests and Profiling

Not applicable. The changes are request validation/lowering and documentation only.

Checklist

• Format changed files with pre-commit.
• Add unit coverage for protocol isolation, active-model filtering, aliases, root-level fields, nested compatibility, and precedence.
• Update public API documentation and affected cookbooks.
• Preserve existing request payload compatibility.
• Follow the SGLang code style guidance.

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CI States

Latest PR Test (Base): ✅ Run #33151263158
Latest PR Test (Extra): ❌ Run #33151262946
Latest PR Test (AMD ROCm 7.2): ⏳ Run #33151263073

[mickqian]

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