[google_maps_flutter] Standardize iOS class and file names

[stuartmorgan-g]

Fixes inconsistencies in the class and file naming in google_maps_flutter_ios:

• Standardizes on FGM instead of a mix of FGM and the older FLT.
• Removes GoogleMaps from most class names, as now that we are using FGM we don't need to further specific that it's related to Google Maps, and removing that makes it much easier to see the useful part of the name.
• Ensures that all files have a prefix matching their class, for consistency.
• Removes prefixing from all test classes and files, since tests don't need prefixes as they aren't part of client builds.

Part of flutter/flutter#102601

Pre-Review Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
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• I linked to at least one issue that this PR fixes in the description above.
• I updated pubspec.yaml with an appropriate new version according to the pub versioning philosophy, or I have commented below to indicate which version change exemption this PR falls under¹.
• I updated CHANGELOG.md to add a description of the change, following repository CHANGELOG style, or I have commented below to indicate which CHANGELOG exemption 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.

Footnotes

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[stuartmorgan-g]

This is another one I should have done before #10852, but I can reconcile the two PRs once one lands. This annoys me every time I try to work in this plugin, and after all the recent time I've spent in it I couldn't take it any more 😐

[gemini-code-assist]

Code Review

This pull request standardizes Objective-C class and file names in the google_maps_flutter_ios package. The changes replace the FLT prefix with FGM, remove the GoogleMaps substring from many class names, and rename files to match their class names. Test classes and files have had prefixes removed, and test utility files have been moved into a TestUtils directory. The package version has been updated and a corresponding entry was added to the changelog.