fix(session): check for context overflow mid-turn in finish-step

[aryasaatvik]

Summary

• fixes #4845
• Check for context overflow during streaming in finish-step event handler using actual token counts from AI SDK
• Break stream loop early and return "compact" when overflow detected
• Handle "compact" return in prompt.ts to trigger SessionCompaction.create immediately

Problem

Context compaction only triggered after an assistant turn completed. When a model made many tool calls with large outputs, context could overflow mid-turn before compaction had a chance to run.

Solution

Move the overflow check from post-turn to mid-turn by checking in the finish-step event handler (which fires after each step). This uses actual token counts from the AI SDK rather than estimates.

Before / After

Before	After

Changes

• processor.ts: Check isOverflow in finish-step, set needsCompaction flag, break loop early, return "compact"
• prompt.ts: Handle "compact" return, trigger SessionCompaction.create, continue loop
• compaction.test.ts: Unit tests for the compaction data flow

Testing

15 tests covering the compaction data flow:

• session.compaction.isOverflow (5 tests) - overflow detection logic
• util.token.estimate (3 tests) - token estimation
• session.getUsage (7 tests) - AI SDK usage normalization to our token format

Tests verify the key units without heavy mocking - if getUsage correctly normalizes AI SDK tokens and isOverflow correctly detects overflow, the integration works.