fix: return cloudrun-sandbox defaults on single-node tier

[ptone]

Summary

• GetDefaultSettingsDataYAML() always returned the workstation template (default_settings.yaml) which defines local (docker) and remote (kubernetes) profiles
• On the Cloud Run single-node tier, this caused "remote (kubernetes)" to appear as the only runtime profile since the local profile gets filtered out
• Added isCloudRunSandboxEnvironment() check to return the cloudrun-sandbox template instead

Changes

• pkg/config/koanf.go: 7-line addition at top of GetDefaultSettingsDataYAML()
• pkg/config/koanf_test.go: 3 new test functions (131 lines) covering positive, negative, and integration scenarios

Test plan

• All pkg/config tests pass including existing TestInitMachine_CloudRunSandbox_* tests
• New tests verify cloudrun-sandbox defaults returned when env conditions met
• New tests verify workstation defaults still returned when conditions NOT met
• Integration test verifies LoadVersionedSettings produces correct profiles on cloudrun-sandbox tier

Tracking: ptone#1448

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[gemini-code-assist]

Code Review

This pull request updates GetDefaultSettingsDataYAML in pkg/config/koanf.go to return the tier-specific Cloud Run Sandbox settings template when running in that environment, and adds corresponding unit tests. However, the reviewer points out that this fix is incomplete because LoadSettingsKoanf relies on GetDefaultSettingsData in pkg/config/init.go, which does not check for the Cloud Run Sandbox environment and will still merge workstation profiles in production.

[gemini-code-assist]

While fixing GetDefaultSettingsDataYAML() correctly addresses the defaults for LoadVersionedSettings(), the main settings loader LoadSettingsKoanf() (defined in koanf.go line 53) calls GetDefaultSettingsData() (defined in init.go line 84) to load the initial embedded defaults.

Currently, GetDefaultSettingsData() does not check isCloudRunSandboxEnvironment() and always returns the workstation defaults (with docker or container runtime) converted to JSON. Because Koanf merges maps, calling LoadSettingsKoanf() in a Cloud Run Sandbox environment will still result in the workstation profiles (local, remote) and runtimes (docker, kubernetes, etc.) being merged into the final Settings struct, even if the global settings file only defines the default profile.

This is also why the new integration test TestGetDefaultSettingsDataYAML_CloudRunSandbox_ProfilesCorrect passes (since it uses LoadVersionedSettings), but the actual runtime configuration loaded via LoadSettingsKoanf in production remains polluted with workstation profiles.

To fully resolve this, GetDefaultSettingsData() in pkg/config/init.go should also be updated to handle the Cloud Run Sandbox environment, converting the sandbox YAML template to JSON.