fix(agent-sessions): count a stamped tool call by the gateway's verdict on the detail page - #1141

🟡 Confidence 3/5 · needs attention
F3 stays open: pre-stamp rows still merge on the page and count per span on the list, and no test covers that divergence.
_{quality 90/100 · 1 warning · tests partial · risk medium}

Makes the detail page read a gateway-stamped tool span by maple_ai.tool_call and its failure by maple_ai.error, the same verdicts the list SQL sums, leaving pre-stamp spans on the old rule. The stamped paths now agree side to side, but the page and the list still disagree for spans ingested before the stamps (F3).

• countedToolCalls counts every gateway-stamped tool span, merging nothing
• spanFailed reads a stamped span's maple_ai.error alone, before the status check
• The summary SQL drops the StatusCode fallback for stamped spans, as the page does

Still open from earlier reviews

• 🟠 Warning · F3 · Session page and list disagree on tool calls for spans ingested before the stamps · packages/agent-sessions/src/session-summary.ts:906

Fixed since the last review

• ✅ F2 · Per-turn toolCalls sums to more than the session's totals row

_{7c4c21f · Updated on every push. Reply "won't fix" to dismiss a finding, or mention @maple to ask about one.}