[google_maps_flutter_web] Fix AdvancedMarker anchors on web

[3ph]

Fixes AdvancedMarker anchor handling in google_maps_flutter_web.

Previously AdvancedMarker.anchor was not applied on web, so custom advanced marker content was positioned as if it used the default anchor. This caused markers with non-default anchors to appear offset from their intended map coordinates.

This PR converts Flutter marker anchor offsets into the Google Maps JavaScript Advanced Marker anchorLeft and anchorTop CSS offset properties when creating marker options, and reapplies those properties when an existing advanced marker is updated.

Adds integration test coverage for creating and updating an AdvancedMarker with custom anchors.

Addresses flutter/flutter#80578.

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• 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.
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[gemini-code-assist]

Code Review

This pull request implements AdvancedMarker anchor handling for the Google Maps Flutter Web plugin. It introduces helper functions to convert anchor offsets to CSS percentage strings and apply them as anchorLeft and anchorTop properties on the underlying AdvancedMarkerElement and its options. Additionally, an integration test has been added to verify that anchors are correctly set and updated. I have no feedback to provide.

[gemini-code-assist]

Code Review

This pull request fixes the anchor handling for AdvancedMarker on the web platform by introducing helper functions to convert anchor offsets to CSS percentages and apply them to AdvancedMarkerElementOptions and AdvancedMarkerElement. It also adds an integration test to verify anchor positioning and updates, and bumps the package version to 0.6.2+4. There are no review comments, and I have no feedback to provide.