feat: track OpenCode sessions (#26128)

This follows #26098 by teaching AI Bridge to read OpenCode session IDs
from the X-OpenCode-Session header, so OpenCode requests are grouped
consistently in interception logs.

Adds unit and integration test coverage for the new header.

<details>
<summary>Coder Agents generated</summary>

This pull request was generated with Coder Agents assistance.
</details>

aibridge/session.go

@@ -75,6 +75,8 @@ func GuessSessionID(client Client, r *http.Request) *string {
 		return cleanRef(r.Header.Get("X-KILOCODE-TASKID"))
 	case ClientCoderAgents:
 		return cleanRef(r.Header.Get("X-Coder-Chat-Id"))
+	case ClientOpenCode:
+		return cleanRef(r.Header.Get("X-OpenCode-Session"))
 	case ClientCrush:
 		return nil // Crush does not send a session ID header.
 	case ClientRoo: