Fix error chaining in getIterationTypesOfIterableSlow

[jakebailey]

Followup to #1059

[Copilot]

Pull Request Overview

This PR fixes error chaining in getIterationTypesOfIterableSlow by switching to the extended diagnostic method and updates the test baselines to match the new error output.

• Replaced checkTypeAssignableTo with checkTypeAssignableToEx to include diagnosticOutput
• Updated iterator and async iterator error baselines to reflect adjusted error counts and messages
• Removed obsolete .diff baseline files

Reviewed Changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
internal/checker/checker.go	Uses checkTypeAssignableToEx instead of checkTypeAssignableTo to chain diagnostics
testdata/baselines/reference/submodule/compiler/iteratorExtraParameters.errors.txt.diff	Removed outdated diff file
testdata/baselines/reference/submodule/compiler/iteratorExtraParameters.errors.txt	Updated error count and message structure
testdata/baselines/reference/submodule/compiler/asyncIteratorExtraParameters.errors.txt.diff	Removed outdated diff file
testdata/baselines/reference/submodule/compiler/asyncIteratorExtraParameters.errors.txt	Updated error count and message structure