fix: bypass ai for singleton-backed viewmodels

[gay00ung]

Summary

This PR makes AI bypass detection more conservative for ViewModels that depend on direct singleton collaborators.

Included changes

• update AI bypass logic to inspect the full ViewModel source file, not only the currently selected changed methods
• keep bypass enabled when:
  • the module does not support kotlinx-coroutines-test
  • and the ViewModel source still contains direct singleton collaborator access
• add a regression test for the case where:
  • the changed method itself does not reference the singleton
  • but another public method in the same file still does

Why this change

d2t auto --ai narrows analysis to changed methods.

That meant singleton-backed ViewModels could still slip into the AI path when the changed method did not directly contain the singleton access, even though the file as a whole was still not a safe AI target.

This PR fixes that mismatch by treating the full ViewModel source as the bypass boundary.

Expected outcome

After this PR is merged:

• singleton-backed ViewModels are more likely to use compile-safe heuristic generation
• auto --ai is less likely to send large unnecessary model requests for unsafe targets
• Android projects with global singleton collaborators should behave more predictably

Validation

• ./gradlew --no-daemon :modules:test-generator:test --tests dev.diff2test.android.testgenerator.OpenAiResponsesTestGeneratorTest

Notes

• this PR does not change provider-specific APIs
• it only tightens the AI-vs-heuristic decision boundary