fix(session): clamp output token count to prevent negative values (#9168)

When reasoningTokens exceeds outputTokens (due to provider SDK inconsistencies),
the subtraction produces a negative output token count. Wrap with Math.max(0, ...)
to ensure non-negative values.

Rebased onto refactored session module structure (session.ts extracted from index.ts).

Author: kagura-agent

packages/opencode/src/session/session.ts

@@ -318,7 +318,7 @@ export const getUsage = (input: {
   const tokens = {
     total,
     input: adjustedInputTokens,
-    output: safe(outputTokens - reasoningTokens),
+    output: Math.max(0, safe(outputTokens - reasoningTokens)),
     reasoning: reasoningTokens,
     cache: {
       write: cacheWriteInputTokens,