[Refactor] Remove prerelease DeepSeek V4.1 config aliases

[hnyls2002]

• Stack on dsv4.1: remaining model and runtime integration #38798 with base dsv4.1. The public DeepSeek-V4.1-Flash config uses the canonical names.
• Remove the three dotted deepseek_v4.1* registrations, five prerelease field aliases, vision_config.max_num_tokens fallback, and DeepseekV41ForConditionalGeneration alias. Old internal checkpoint configs must be migrated to the official names.
• Let the canonical vision config inherit construction and serialization from PretrainedConfig after removing its legacy conversion.
• Preserve canonical nested config flattening, vision field mapping, DeepseekV41ForCausalLM routing, config overrides, and save/reload behavior. Keep the existing V4/V3.2 registrations.
• Validation: 144 passed, 1 skipped, 25 subtests. The unmodified public config produces identical full config snapshots before and after this cleanup for main/DSpark draft construction, nested overrides, save/reload, text/vision subconfigs, and V4/V3.2 registration. Formatting and lint checks passed. GPU weight loading was not rerun for this cleanup.

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CI States

Latest PR Test (Base): ❌ Run #34534472878
Latest PR Test (Extra): ❌ Run #34534472729
Latest PR Test (AMD ROCm 10): ❌ Run #34534472990