feat: add uncertainty visualization with token-level logprobs

[AlexCheema]

Motivation

Adds uncertainty visualization to the chat interface, allowing users to see token-level confidence scores and regenerate responses from any point in the generation. This enables users to:

• Understand model confidence at each token
• Explore alternative completions by regenerating from uncertain tokens
• Debug and analyze model behavior

Changes

Uncertainty Visualization

• Add TokenHeatmap component showing token-level probability coloring
• Toggle uncertainty view per message with bar chart icon
• Display tooltip with probability, logprob, and top alternative tokens on hover

Regenerate from Token

• Add "Regenerate from here" button in token tooltip
• Use continue_final_message in chat template to continue within same turn (no EOS tokens)
• Add continue_from_prefix flag to ChatCompletionTaskParams

Request Cancellation

• Add AbortController to cancel in-flight requests when regenerating mid-generation
• Handle BrokenResourceError server-side when client disconnects gracefully

Additional APIs

• Add Claude Messages API support (/v1/messages)
• Add OpenAI Responses API support (/v1/responses)

Why It Works

• Proper continuation: Using continue_final_message=True instead of add_generation_prompt=True keeps the assistant turn open, allowing the model to continue naturally from the prefix without end-of-turn markers
• Clean cancellation: AbortController aborts the HTTP request, and server catches BrokenResourceError to avoid crashes
• Stable hover during generation: TokenHeatmap tracks hover by index (stable across re-renders) with longer hide delay during generation

Test Plan

Manual Testing

• Send a message and verify logprobs are collected
• Enable uncertainty view and verify token coloring based on probability
• Hover over tokens to see tooltip with alternatives
• Click "Regenerate from here" on a token mid-response
• Verify the response continues naturally from that point
• Verify aborting mid-generation and regenerating works without server crash

Automated Testing

• Added tests for Claude Messages API adapter
• Added tests for OpenAI Responses API adapter

🤖 Generated with Claude Code