[camera_avfoundation] Fix Swift analyzer issue _captureSessionQueue - #13332

I don't know why these analyzer issues are being reported one-at-a-time in CI on #13284 instead of all at once, but #13332 was only one of a few analyzer issues:
```
darwin/Tests/CameraSessionPresetsTests.swift:28:9: error: variable 'currentFormat' was never mutated; consider changing to 'let' constant
    var currentFormat: CaptureDeviceFormat = captureFormatMock
    ~~~ ^
    let
darwin/Tests/CameraInitRaceConditionsTests.swift:62:24: warning: immutable value '_captureSessionQueue' was never used; consider replacing with '_' or removing it [#NoUsage]
60 | 
61 |   func testFlutterChannelInitializedWhenStartingImageStream() {
62 |     let (cameraPlugin, _captureSessionQueue) = createCameraPlugin()
darwin/Tests/AvailableCamerasTests.swift:49:11: warning: variable 'requiredTypes' was never mutated; consider changing to 'let' constant [#VariableNeverMutated]
 47 |       telephotoCamera.position = .back
 48 | 
 49 |       var requiredTypes: [AVCaptureDevice.DeviceType] = [
darwin/Tests/AvailableCamerasTests.swift:52:11: warning: variable 'cameras' was never mutated; consider changing to 'let' constant [#VariableNeverMutated]
 50 |         .builtInWideAngleCamera, .builtInTelephotoCamera, .builtInUltraWideCamera,
 51 |       ]
 52 |       var cameras = [wideAngleCamera, frontFacingCamera, telephotoCamera, ultraWideCamera]
darwin/Tests/AvailableCamerasTests.swift:86:11: warning: variable 'requiredTypes' was never mutated; consider changing to 'let' constant [#VariableNeverMutated]
 84 |       frontFacingCamera.position = .front
 85 | 
 86 |       var requiredTypes: [AVCaptureDevice.DeviceType] = [
darwin/Tests/AvailableCamerasTests.swift:118:11: warning: variable 'requiredTypes' was never mutated; consider changing to 'let' constant [#VariableNeverMutated]
116 |       unspecifiedCamera.position = .unspecified
117 | 
118 |       var requiredTypes: [AVCaptureDevice.DeviceType] = [
darwin/Tests/CameraOrientationTests.swift:120:14: warning: weak variable 'weakDevice' was never mutated; consider changing to 'let' constant [#VariableNeverMutated::WeakMutability]
118 |     let captureSessionQueue = DispatchQueue(label: "capture_session_queue")
119 |     weak var weakPlugin: CameraPlugin?
120 |     weak var weakDevice = mockDevice
darwin/Tests/CameraOrientationTests.swift:157:14: warning: weak variable 'weakEventAPI' was never mutated; consider changing to 'let' constant [#VariableNeverMutated::WeakMutability]
155 |     }
156 | 
157 |     weak var weakEventAPI = mockEventAPI

```

This wasn't showing up in CI due to flutter/flutter#194517.

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flutter/packages@10a178c...618afba

2026-10-20 [email protected] [pigeon] add version checking for
objective_c and jni dependencies (flutter/packages#13283)
2026-10-19 [email protected] [go_router_builder] Adopt
shared analyzer helpers from source_helper: ^1.4.0
(flutter/packages#13337)
2026-10-19 [email protected] [pigeon] fix instance manager test flake
(flutter/packages#13303)
2026-10-19 [email protected] [vector_graphics] Don't report a handled
loading error as unhandled (flutter/packages#13315)
2026-10-19 [email protected] [file_selector_macos] Drop deprecated
allowedFileTypes code path (flutter/packages#13302)
2026-10-19 [email protected] [shared_preferences_android] Update
androidx.datastore:datastore to 1.2.1 (flutter/packages#13334)
2026-10-19 [email protected] [in_app_purchase_storekit] Suppress
StoreKit 1 deprecation warnings (flutter/packages#13301)
2026-10-19 [email protected] [camera_avfoundation] Fix Swift analyzer
issue _captureSessionQueue (flutter/packages#13332)
2026-10-19 [email protected] [material_ui]
Unskip web tests that pass on CanvasKit and Skwasm: color_scheme_test,
elevated_button_test, filled_button_test, icon_button_test
(flutter/packages#13327)
2026-10-19 [email protected] [in_app_purchase_android] Update
to Play Billing Library 9.1.0 (flutter/packages#12781)
2026-10-19 [email protected] [webview_flutter_wkwebview] Require iOS 15
/ macOS 12 and drop deprecated SecTrustGetCertificateAtIndex code path
(flutter/packages#13294)

If this roll has caused a breakage, revert this CL and set the roller
to dry run mode using the controls here:
https://autoroll.skia.org/r/flutter-packages-flutter-autoroll
Please CC [email protected] on the revert to ensure that a
human
is aware of the problem.

To file a bug in Flutter:
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flutter/packages@10a178c...618afba

2026-10-20 [email protected] [pigeon] add version checking for
objective_c and jni dependencies (flutter/packages#13283)
2026-10-19 [email protected] [go_router_builder] Adopt
shared analyzer helpers from source_helper: ^1.4.0
(flutter/packages#13337)
2026-10-19 [email protected] [pigeon] fix instance manager test flake
(flutter/packages#13303)
2026-10-19 [email protected] [vector_graphics] Don't report a handled
loading error as unhandled (flutter/packages#13315)
2026-10-19 [email protected] [file_selector_macos] Drop deprecated
allowedFileTypes code path (flutter/packages#13302)
2026-10-19 [email protected] [shared_preferences_android] Update
androidx.datastore:datastore to 1.2.1 (flutter/packages#13334)
2026-10-19 [email protected] [in_app_purchase_storekit] Suppress
StoreKit 1 deprecation warnings (flutter/packages#13301)
2026-10-19 [email protected] [camera_avfoundation] Fix Swift analyzer
issue _captureSessionQueue (flutter/packages#13332)
2026-10-19 [email protected] [material_ui]
Unskip web tests that pass on CanvasKit and Skwasm: color_scheme_test,
elevated_button_test, filled_button_test, icon_button_test
(flutter/packages#13327)
2026-10-19 [email protected] [in_app_purchase_android] Update
to Play Billing Library 9.1.0 (flutter/packages#12781)
2026-10-19 [email protected] [webview_flutter_wkwebview] Require iOS 15
/ macOS 12 and drop deprecated SecTrustGetCertificateAtIndex code path
(flutter/packages#13294)

If this roll has caused a breakage, revert this CL and set the roller
to dry run mode using the controls here:
https://autoroll.skia.org/r/flutter-packages-flutter-autoroll
Please CC [email protected] on the revert to ensure that a
human
is aware of the problem.

To file a bug in Flutter:
https://github.com/flutter/flutter/issues/new/choose

To report a problem with the AutoRoller itself, please file a bug:
https://issues.skia.org/issues/new?component=1389291&template=1850622

Documentation for the AutoRoller is here:
https://skia.googlesource.com/buildbot/+doc/main/autoroll/README.md