events: pair every model_call_started with a model_call_finished

[justrach]

Fixes #1665.

Root cause

agent_request.request emitted model_call_started at the top of its rebuild loop. A request rebuilt before its response (a WebSocket reanchor, a stream reconnect, a steer, a recovered overflow) emitted another model_call_started without closing the attempt before it. A request that ended with an error closed nothing.

Fix

• New model_call.Call tracks the open attempt.
• A rebuilt request closes the attempt with ok: false, retried: true before the next one starts.
• An error return closes it (errdefer) with ok: false, retried: false.
• Successful attempts emit the same event as before.
• The --schema events and the SDK event types document the optional retried field.

Not changed

Recovery is still reported as transport detail rather than a typed retry event, and there is no attempt-budget control. Both are separate from pairing.

Tests

• New: two starts close the first as retried, a double finish emits once, and a failure closes with retried: false.
• Model-call, schema and request unit tests: 199 pass.
• Tier-1 pre-push gate is green.