[camera_avfoundation] Fix Swift 6 Sendability warning in CaptureDevice protocol

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Description

Fixes a Swift 6 Sendability warning produced when building camera_avfoundation with SWIFT_STRICT_CONCURRENCY=targeted or higher (the Xcode 16+ default).

Root cause

CaptureDevice.swift declares:

func setExposureTargetBias(
  _ bias: Float, completionHandler handler: ((CMTime) -> Void)?)

AVCaptureDevice (the Apple system type that conforms to this protocol) exposes the same method with a @Sendable closure:

func setExposureTargetBias(
  _ bias: Float, completionHandler handler: (@Sendable (CMTime) -> Void)?)

The missing @Sendable annotation on the protocol requirement creates a Sendability mismatch. Under SWIFT_STRICT_CONCURRENCY=targeted this is a warning; in Swift 6 language mode it is a hard error.

Fix

Three files changed:

1. CaptureDevice.swift — adds @Sendable to the completion handler in the protocol declaration to match AVFoundation's signature.
2. MockCaptureDevice.swift — updates the mock to match the new protocol signature (both the stub property type and the method).
3. CameraExposureTests.swift — adds testSetExposureTargetBias_completionHandlerIsInvoked, which:
   • Exercises the previously-untested non-nil completion handler path
   • Serves as a compile-time regression guard: MockCaptureDevice conforms to CaptureDevice, so if @Sendable is removed from the protocol the mock conformance breaks at compile time

Tests

• Added testSetExposureTargetBias_completionHandlerIsInvoked in CameraExposureTests.swift
• Updated MockCaptureDevice stub and method signatures to use @Sendable

Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the Tree Hygiene page, which explains my responsibilities.
• I read and followed the Flutter Style Guide, including Features we expect every widget to implement.
• I signed the CLA.
• I listed at least one issue that this PR fixes in the description above.
• I updated/added relevant documentation.
• I added new tests or all pre-existing tests are still passing.
• I didn't break the roll

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[gemini-code-assist]

Code Review

This pull request updates the CaptureDevice protocol in camera_avfoundation by adding the @Sendable attribute to the setExposureTargetBias completion handler, along with a version bump to 0.10.1+1. Feedback indicates that this change will cause compilation failures in mock implementations that have not been updated to match the new protocol signature.

[gemini-code-assist]

This change to the protocol requirement will cause a compilation error in MockCaptureDevice.swift (and any other implementations of this protocol) because the method signature must match the protocol requirement exactly, including the @Sendable attribute. The PR description's claim that mocks are not affected is incorrect; they must be updated to include @Sendable in both the method signature and the setExposureTargetBiasStub property type to maintain protocol conformance.