Fix null safety in response array loops and OpenCode method reference

[rajbos]

Problem

Three distinct WARNING log entries appeared during backend sync:

1. TypeError: tokenTracker.getOpenCodeSessionData is not a function — fired for every OpenCode session file during sync.
2. TypeError: Cannot read properties of undefined (reading 'value') — from getModelUsageFromSession when processing VS Code Insiders JSONL delta-format files.
3. TypeError: Cannot read properties of undefined (reading 'kind') — from analyzeSessionUsage / trackEnhancedMetrics for the same files.

Root Causes

1. Wrong receiver for getOpenCodeSessionData (extension.ts)

The backend facade wired getOpenCodeSessionData by calling it on tokenTracker directly, but the method lives on tokenTracker.openCode (the OpenCodeDataAccess instance):

// Before (broken)
(tokenTracker as any).getOpenCodeSessionData(sessionFile)

// After (fixed)
(tokenTracker as any).openCode.getOpenCodeSessionData(sessionFile)

2. Null entries in reconstructed response arrays (usageAnalysis.ts)

VS Code Copilot Chat now stores some sessions as JSONL delta streams ({"kind":0/1/2,...}). When the applyDelta reconstruction rebuilds requests[].response, the resulting array can contain null/undefined entries. Multiple loops iterated over these arrays and accessed .kind or .value without guarding for nulls.

Fixes

File	Change
extension.ts	Route getOpenCodeSessionData through tokenTracker.openCode
usageAnalysis.ts	Add if (!responseItem) continue guard in 5 response-array loops (analyzeSessionUsage delta-path x2, JSON-path x1, trackEnhancedMetrics x2)
usageAnalysis.ts	Use responseItem?.value (optional chain) instead of responseItem.value in getModelUsageFromSession (x2)

Notes on other logged messages

• "spans 2 days" — informational log() messages (not warnings), correctly noting a session file spans midnight. No change needed.
• type=undefined — debug log for the first 3 lines of today's JSONL files; VS Code delta-format events use kind not type, so event.type is undefined. The sync service already skips those lines harmlessly. No change needed.