[Misc] add multi modal model fallback in generic model config parse

[pallasathena92]

What this PR does

• Enhance GenericModelConfig fallback to probe nested sub-configs (text_config, llm_config, language_config) commonly found in multimodal models
• Add MoE-aware parameter estimation for unregistered Mixture-of-Experts models
• Move estimateMoEParams() from deepseek_vl.go to interface.go as a shared utility

Why we need it

When an unregistered multimodal model (e.g., composite vision-language-audio models) falls back to GenericModelConfig, critical fields like hidden_size, max_position_embeddings, and transformers_version are
missed because they are nested under keys like llm_config, text_config, or language_config. This causes:

• GetContextLength() → 0
• GetParameterCount() → 0 (or massively underestimated for MoE models)
• GetTransformerVersion() → ""
• HasVision() → false

What changes in this PR

pkg/hfutil/modelconfig/interface.go

• Add MaxSequenceLength, MoE fields (NRoutedExperts, NSharedExperts, MoeIntermediateSize), and hasVisionConfig to GenericModelConfig
• Add probeNestedConfig() helper that extracts fields from nested sub-configs and detects vision_config presence
• Override GetContextLength() to fall back to MaxSequenceLength
• Override HasVision() to return detected vision config presence
• Override GetParameterCount() to use MoE-aware estimation when experts are detected
• Move estimateMoEParams() here from deepseek_vl.go (general-purpose, not DeepSeek-specific)
• Call probeNestedConfig() unconditionally in loadGenericModelConfig() (safe — only fills zero-valued fields)

pkg/hfutil/modelconfig/deepseek_vl.go

• Remove estimateMoEParams() (moved to interface.go, same package — no behavior change)

pkg/hfutil/modelconfig/interface_test.go

• TestGenericMultimodalFallback — nested llm_config + vision_config with MoE fields
• TestGenericMultimodalFallback_TextConfig — text_config variant
• TestGenericMultimodalFallback_LanguageConfig — language_config variant
• TestGenericMoEFallback — flat MoE config with num_local_experts field name variant
• TestGenericContextLengthMaxSequenceLength — max_sequence_length fallback

Fixes #

How to test

Checklist

• Tests added/updated (if applicable)
• Docs updated (if applicable)
• make test passes locally