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 {