refactor: cleanup platform mocking

[vaind]

• refactor: cleanup platform mocking

📜 Description

part of #2646

💡 Motivation and Context

💚 How did you test it?

📝 Checklist

• I reviewed submitted code
• I added tests to verify changes
• No new PII added or SDK only sends newly added PII if sendDefaultPii is enabled
• I updated the docs if needed
• All tests passing
• No breaking changes

🔮 Next steps

[codecov]

Codecov Report

Attention: Patch coverage is 88.88889% with 5 lines in your changes missing coverage. Please review.

Project coverage is 87.21%. Comparing base (0f8bd48) to head (8411978).

Files with missing lines	Patch %	Lines
dart/lib/src/platform/_io_platform.dart	72.72%	3 Missing ⚠️
dart/lib/src/platform/platform.dart	83.33%	1 Missing ⚠️
dart/lib/src/platform_checker.dart	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##               v9    #2730      +/-   ##
==========================================
+ Coverage   87.16%   87.21%   +0.05%     
==========================================
  Files         175      176       +1     
  Lines        5966     5985      +19     
==========================================
+ Hits         5200     5220      +20     
+ Misses        766      765       -1     

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[github-actions]

	Fails
🚫	Please consider adding a changelog entry for the next release.

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Please add an entry to CHANGELOG.md to the "Unreleased" section. Make sure the entry includes this PR's number.

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## Unreleased

- cleanup platform mocking ([#2730](https://github.com/getsentry/sentry-dart/pull/2730))

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Generated by 🚫 dangerJS against 8411978

[ueman]

Isn't the platform checker used to decide which native integration is called? So actually returning true for 'isMacOs' on web would cause the macos integrations to be called on web?

[vaind]

Isn't the platform checker used to decide which native integration is called? So actually returning true for 'isMacOs' on web would cause the macos integrations to be called on web?

I believe that was the case in the past but looking at how the code which calls hasNativeIntegration has evolved, it seems pretty useless now. Also, it's actually always true for the platforms we support:

  bool get hasNativeIntegration =>
      platform.isWeb ||
      platform.isAndroid ||
      platform.isIOS ||
      platform.isMacOS ||
      platform.isWindows ||
      platform.isLinux;
      ```

[vaind]

Error: A value of type 'SentryTransaction Function(SentryTransaction)' can't be assigned to a variable of type 'FutureOr<SentryTransaction?> Function(SentryTransaction, Hint)?'.
 - 'SentryTransaction' is from 'package:sentry/src/protocol/sentry_transaction.dart' ('../dart/lib/src/protocol/sentry_transaction.dart').
 - 'Hint' is from 'package:sentry/src/hint.dart' ('../dart/lib/src/hint.dart').
      fixture.options.beforeSendTransaction = (transaction) {
                                              ^
Shuffling test order with --test-randomize-ordering-seed=3637785904

doesn't seem related to the changes in this PR

[buenaflor]

we recently added hints to transactions, looks like flutter tests weren't updated

cc @denrase