fix(desktop): prevent thinking block from closing mid-streaming

[stremtec]

Summary

Fixes a bug where the Thinking/Reasoning disclosure block auto-closes while reasoning text is still actively streaming. Reported as: block closes suddenly when thinking text gets long enough, during active streaming.

Root Cause

ReasoningAccordionGroup checks whether any reasoning part in a fixed range [startIndex, endIndex) is still pending:

s.message.parts
  .slice(startIndex, Math.min(s.message.parts.length, endIndex))
  .some(p => p?.type === 'reasoning' && p.status?.type !== 'complete')

During streaming, new reasoning parts can be appended to s.message.parts beyond endIndex (which was fixed at component mount time). When the original part completes and a new part starts beyond the range, .some() returns false → pending = false → block closes.

Fix

Remove the endIndex cap from slice() — check all parts from startIndex onward:

s.message.parts
  .slice(Math.max(0, startIndex))  // no endIndex cap
  .some(p => p?.type === 'reasoning' && p.status?.type !== 'complete')

Since startIndex still scopes to this reasoning group's beginning, and the p?.type === 'reasoning' filter excludes non-reasoning parts (tool calls, etc.), removing the end bound is safe — newly appended reasoning parts are now correctly detected.

Verification

• TypeScript compiles cleanly
• Existing behavior unchanged when no new parts are appended