[webview_flutter_wkwebview] Retain registrar for WebKit callbacks - #13140

The handwritten WebKit callback implementations currently keep an unowned
reference to their ProxyAPIRegistrar. WebKit can retain those callback objects
after the plugin owner has been torn down, so a late callback can dereference a
deallocated registrar and abort the process.

This change keeps the registrar alive for the lifetime of each callback object.
Plugin teardown still clears the instance manager before releasing its registrar,
which breaks registrar/callback cycles owned by the plugin.

Part of flutter/flutter#192975. This complements #13130, which handles the
separate generated-codec lifetime during in-flight encoding.

Tests

• Swift format and strict lint for webview_flutter_wkwebview
• iOS example build
• iOS native unit tests: 147 passed
• Dart tests: 158 passed
• Dart and iOS analysis

Pre-Review Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
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• I read and followed the relevant style guides and ran the auto-formatter.
• I signed the CLA.
• The title of the PR starts with the name of the package surrounded by square brackets, e.g. [shared_preferences]
• I linked to at least one issue that this PR fixes in the description above.
• I followed the version and CHANGELOG instructions, using semantic versioning and the repository CHANGELOG style, or I have commented below to indicate which documented exception this PR falls under¹.
• I updated/added any relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or I have commented below to indicate which test exemption this PR falls under¹.
• All existing and new tests are passing.

No public API or documentation changes are needed for this ownership fix.

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