Refactor TaskPropertyAccessor to absorb cancellation token lifecycle

[onevcat]

Summary

KingfisherWrapper.setImage(...) currently passes previousCancellationToken and setCancellationToken as standalone parameters in addition to TaskPropertyAccessor.

These token-related parameters belong to the same task lifecycle concern as:

• setTaskIdentifier
• getTaskIdentifier
• setTask

The current shape works, but it creates parameter sprawl and makes the helper signature harder to read.

Proposed Direction

Fold cancellation-token access into TaskPropertyAccessor so the helper receives a single lifecycle accessor instead of splitting task state across:

• taskAccessor
• previousCancellationToken
• setCancellationToken

Possible shape:

• add token getter/setter accessors to TaskPropertyAccessor
• let the helper cancel the previous token and store the new token through that accessor

Scope

This is a readability / maintainability cleanup only.
It should not change behavior.

Non-Goals

• do not change the current stale-disk-cache semantics
• do not refactor unrelated associated-object patterns in this issue
• do not change public API surface

Affected Files

Likely at least:

• Sources/Extensions/HasImageComponent+Kingfisher.swift
• Sources/Extensions/ImageView+Kingfisher.swift
• Sources/Extensions/NSTextAttachment+Kingfisher.swift
• Sources/Extensions/CPListItem+Kingfisher.swift
• Sources/Extensions/UIButton+Kingfisher.swift
• Sources/Extensions/NSButton+Kingfisher.swift

Acceptance

• helper signature becomes simpler
• token lifecycle stays encapsulated with other task lifecycle accessors
• no behavior change
• existing stale cache tests continue to pass