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/internal/integrationtest/bridge_internal_test.go

@@ -1049,6 +1049,16 @@ func TestSessionIDTracking(t *testing.T) {
 				"User-Agent": []string{"Zed/0.219.4+stable.119.abc123 (macos; aarch64)"},
 			},
 		},
+		{
+			name:            "opencode",
+			fixture:         fixtures.AntSimple,
+			expectedClient:  aibridge.ClientOpenCode,
+			expectSessionID: "ses_15a48edefffe7oY0YcIHRv29dD",
+			header: http.Header{
+				"User-Agent":         []string{"opencode/1.16.0 ai-sdk/provider-utils/4.0.23 runtime/bun/1.3.14"},
+				"X-OpenCode-Session": []string{"ses_15a48edefffe7oY0YcIHRv29dD"},
+			},
+		},
 	}
 
 	for _, tc := range testCases {

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:

aibridge/session_test.go

@@ -182,6 +182,23 @@ func TestGuessSessionID(t *testing.T) {
 			name:   "coder_agents_without_chat_id",
 			client: aibridge.ClientCoderAgents,
 		},
+		// OpenCode.
+		{
+			name:      "opencode_with_session_header",
+			client:    aibridge.ClientOpenCode,
+			headers:   map[string]string{"X-OpenCode-Session": "ses_15a48edefffe7oY0YcIHRv29dD"},
+			sessionID: utils.PtrTo("ses_15a48edefffe7oY0YcIHRv29dD"),
+		},
+		{
+			name:      "opencode_with_whitespace_in_header",
+			client:    aibridge.ClientOpenCode,
+			headers:   map[string]string{"X-OpenCode-Session": "  ses_15a48edefffe7oY0YcIHRv29dD  "},
+			sessionID: utils.PtrTo("ses_15a48edefffe7oY0YcIHRv29dD"),
+		},
+		{
+			name:   "opencode_without_session_header",
+			client: aibridge.ClientOpenCode,
+		},
 		// Crush.
 		{
 			name:   "crush_returns_empty",