fix: prevent nil pointer crash in OpenAI SSE tool call accumulation

[duhd-vnpay]

Summary

• Root cause fix: openai.go — accumulators map (keyed by SSE tool_call index) was iterated with for i := 0; i < len(map); i++, assuming contiguous keys 0,1,2... SSE indices from extended thinking models can be non-contiguous (e.g. {0, 2}), causing accumulators[i] → nil → panic on acc.rawArgs
• Defense: lanes.go — add defer recover() in scheduler lane goroutine so panics log error + return semaphore token instead of crashing the process
• Cleanup: tracing.go + gateway.go — add SweepOrphanTraces() to mark stuck running traces as error on startup (traces older than 1h with status running = orphan from crash)

Changed files

File	Change
internal/providers/openai.go	Iterate map by sorted keys instead of sequential 0..len-1
internal/scheduler/lanes.go	Add recover() in goroutine defer
internal/store/tracing_store.go	Add SweepOrphanTraces to interface
internal/store/pg/tracing.go	Implement SweepOrphanTraces (UPDATE running→error)
cmd/gateway.go	Call sweep on startup in background goroutine

Test results (18 tests, all PASS)

providers (9/9 PASS):

• ContiguousToolCalls — indices 0,1 (happy path)
• NonContiguousToolCallIndices — indices 0,2 skip 1 (the crash scenario)
• LargeGapToolCallIndices — indices 0,5,10
• SingleToolCallAtHighIndex — single call at index 3
• ToolCallWithThoughtSignature — metadata preserved
• TextOnly / EmptyToolCalls — no tool calls
• HTTPError — 429 → proper HTTPError
• CancelledContext — context cancel → error

scheduler (5/5 PASS):

• NoPanic — normal operation
• PanicRecovery — panic → lane recovers, accepts new work
• MultiplePanics — 3 panics → all semaphore tokens returned
• CancelledContext — cancelled ctx with full lane → error
• StatsAfterPanic — active counter decremented after panic

store/pg (4/4 PASS, integration with PostgreSQL):

• SweepsOldRunning — running trace 2h old → swept
• IgnoresRecentRunning — running trace 5min old → NOT swept
• IgnoresCompleted — completed trace → NOT swept
• NoOrphans — no running traces → swept=0

🤖 Generated with Claude Code