Expand CI validation to cover the supported toolchain and platform matrix with no_legacy_features

[nradakovic]

What

Update CI so that all supported toolchain and platform configurations are validated with the explicit-feature model and no_legacy_features enabled.

The goal is to ensure the migration is enforced through automation and remains stable over time.

Acceptance Criteria (DoD)

• CI validates the supported Linux and QNX configuration matrix with no_legacy_features enabled.
• The CI matrix reflects the repository's supported toolchain and platform claims.
• Regressions caused by missing or misconfigured explicit features fail CI.
• CI coverage is sufficient to give maintainers confidence that supported scenarios remain functional.

How

• Define the supported validation matrix using the repository's current example and configuration coverage as the baseline.
• Update CI jobs or workflows to run the explicit-feature configuration.
• Include representative host, cross, runtime-specific, and QNX scenarios.
• Align CI validation scope with the supported configurations documented and exercised in examples.
• Ensure failure output identifies the regressed configuration or feature area.

Out of Scope

• Supporting configurations that are not officially supported by the repository.
• Replacing the example workspace with a different validation framework.
• Non-toolchain CI cleanup unrelated to this migration.