## What changed
Map provider `budget_reached` errors to `SessionBudgetExceeded` for HTTP 400 responses and streaming failures. End SSE streams immediately on this error, ignoring retry headers and later completion events, and display `budget reached` in the TUI.
## Testing
Add coverage for HTTP error classification, immediate SSE termination without retries, WebSocket error delivery and subsequent request recovery, and TUI error rendering and task-state cleanup.
GitOrigin-RevId: 6a62d699cdb9cb446214cacfacd0c9ed68bf21a6