fix(resources): support multiple named devcontainer configurations in auto-discovery

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

features/steps/devcontainer_handler_steps.py

@@ -256,3 +256,58 @@ def step_check_is_trigger(context: Context, rtype: str) -> None:
 @then('"{rtype}" should not be a trigger type')
 def step_check_not_trigger(context: Context, rtype: str) -> None:
     assert is_trigger_type(rtype) is False
+
+
+# ── Given steps for named configurations ─────────────────────
+
+
+@given('the directory has a named devcontainer configuration "{name}"')
+def step_add_named_dc(context: Context, name: str) -> None:
+    tmp = Path(context.tmp_path)
+    named_dir = tmp / ".devcontainer" / name
+    named_dir.mkdir(parents=True, exist_ok=True)
+    (named_dir / "devcontainer.json").write_text(_VALID_DC_JSON, encoding="utf-8")
+
+
+@given('the directory has a named devcontainer configuration "{name}" with invalid JSON')
+def step_add_named_dc_invalid(context: Context, name: str) -> None:
+    tmp = Path(context.tmp_path)
+    named_dir = tmp / ".devcontainer" / name
+    named_dir.mkdir(parents=True, exist_ok=True)
+    (named_dir / "devcontainer.json").write_text("not valid json {{", encoding="utf-8")
+
+
+@given("the directory has an empty .devcontainer directory")
+def step_add_empty_dc_dir(context: Context) -> None:
+    tmp = Path(context.tmp_path)
+    dc_dir = tmp / ".devcontainer"
+    dc_dir.mkdir(exist_ok=True)
+
+
+# ── Then steps for named configurations ──────────────────────
+
+
+@then('the first result should have config name "{name}"')
+def step_check_first_result_config_name(context: Context, name: str) -> None:
+    assert len(context.discovery_results) > 0
+    result = context.discovery_results[0]
+    assert result.config_name == name, (
+        f"Expected config_name={name!r}, got {result.config_name!r}"
+    )
+
+
+@then("the first result should have no config name")
+def step_check_first_result_no_config_name(context: Context) -> None:
+    assert len(context.discovery_results) > 0
+    result = context.discovery_results[0]
+    assert result.config_name is None, (
+        f"Expected config_name=None, got {result.config_name!r}"
+    )
+
+
+@then('the result config names should include "{name}"')
+def step_check_result_config_names_include(context: Context, name: str) -> None:
+    names = {r.config_name for r in context.discovery_results}
+    assert name in names, (
+        f"Expected {name!r} in config names, got {names!r}"
+    )