Support GPT-5.6 tool calls in Daisen

[syifan]

Summary

• send reasoning_effort: "none" on GPT-5.6 Chat Completions turns that offer function tools
• recognize provider-prefixed, tagged, snapshot, and fine-tuned GPT-5.6 model IDs
• surface the provider error when Daisen's compatibility fallback retries a turn without tools
• make gpt-5.6-sol the OpenAI preset and new-configuration default while preserving saved user selections
• add focused backend/frontend regression tests and document the compatibility behavior

Root cause

OpenAI's /v1/chat/completions endpoint returns HTTP 400 when GPT-5.6 function tools are combined with the model's default reasoning effort:

Function tools with reasoning_effort are not supported ... use /v1/responses or set reasoning_effort to 'none'.

Daisen then exercised its capability fallback and retried without tools, which made GPT-5.6 appear to answer normally while never invoking a tool. GPT-5.5 did not reject the original request.

Daisen currently standardizes on the OpenAI-compatible Chat Completions protocol, so this PR applies the endpoint-recommended reasoning_effort: "none" setting only to GPT-5.6 tool turns. Supporting GPT-5.6 reasoning and tools together would require a future Responses API provider implementation.

The no-tools capability fallback remains for OpenAI-compatible gateways, but it is no longer silent: the chat now reports the rejected tool request and provider error before retrying.

Validation

• go test ./daisen2/...
• npm test in daisen2/static — 55 passed
• npm run build in daisen2/static
• live synthetic API comparison:
  • GPT-5.5: tool call succeeds unchanged
  • GPT-5.6 Luna/Sol/Terra: all reject the old payload and all call the tool with reasoning_effort: "none"
• browser verification against the supplied page-migration trace:
  • GPT-5.6 Sol made one data_query call and returned synthetic value 42
  • GPT-5.5 made one data_query call and returned synthetic value 43
  • a fresh configuration selects gpt-5.6-sol in the OpenAI settings panel
  • no browser console errors
  • no trace-table data was sent to the external model

Fixes #463