Extend DevcontainerDiscoveryResult and discover_devcontainers() to scan
.devcontainer/<name>/devcontainer.json patterns (one subdirectory level)
in addition to the existing fixed paths. Each named configuration produces
a distinct result with config_name set to the subdirectory name (e.g.
'api', 'frontend'). Root-level configs retain config_name=None.
- Replace _SCAN_PATHS with _FIXED_SCAN_PATHS for root-level configs
- Add glob-based scan of .devcontainer/<name>/devcontainer.json
- Add config_name: str | None attribute to DevcontainerDiscoveryResult
- Validate config_name (must be non-empty str or None)
- Add 8 new Behave scenarios covering named, multiple, mixed, empty cases
- Add 4 new Robot Framework integration tests for named config discovery
- All existing tests continue to pass (no regression)
ISSUES CLOSED: #2615
freemo