[google_maps_flutter] Convert marker controllers to Swift

[stuartmorgan-g]

This converts MarkerController and ClusterManagersController to Swift in the _sdk* packages.

The remaining Obj-C code will be migrated in a series of follow-up PRs to keep the scope of each PR reasonable for review.

The conversion process was:

• Initial conversion via Gemini, with explicit instruction to keep the structure the same.
• Side-by-side manual review of the old and new versions of the files.
• Manual fixes and improvements.

Part of flutter/flutter#119108

Pre-Review Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the AI contribution guidelines and understand my responsibilities, or I am not using AI tools.
• I read the Tree Hygiene page, which explains my responsibilities.
• 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.

Footnotes

1. Regular contributors who have demonstrated familiarity with the repository guidelines only need to comment if the PR is not auto-exempted by repo tooling. ↩ ↩²

[gemini-code-assist]

Code Review

This pull request converts the marker and cluster manager controllers from Objective-C to Swift across the google_maps_flutter_ios_sdk10, google_maps_flutter_ios_sdk9, and shared iOS codebases. The feedback recommends aligning the opening braces of function declarations to the same line in ConversionUtils.swift to adhere to Swift style guidelines. Additionally, it suggests simplifying the extraction of marker identifiers in pigeonCluster by replacing the filter and force-cast chain with a single, safer compactMap operation.