Add unified declarative flow loading (#6308)

Add a single declaration loader shared by API and CLI callers.

- Add FlowDefinition.from_declaration for FlowDefinition instances, dictionaries, YAML/JSON strings, and file paths
- Add Flow.from_declaration to build runnable flows directly from the same inputs
- Route declarative flow CLI loading through Flow.from_declaration so path handling and validation stay centralized

```
# Load just the serializable definition when you do not need to run it yet.
definition = FlowDefinition.from_declaration(path="flows/research.crewai")
definition = FlowDefinition.from_declaration(contents=flow_yaml)
definition = FlowDefinition.from_declaration(contents=flow_dict)

# Build a runnable flow directly from the same declaration inputs.
flow = Flow.from_declaration(path="flows/research.crewai")
flow = Flow.from_declaration(contents=flow_yaml)
flow = Flow.from_declaration(contents=flow_dict)
flow = Flow.from_declaration(contents=definition)

# Run it like any other flow.
result = flow.kickoff(inputs={"topic": "AI agents"})

# The CLI now goes through the same path-based loader.
# crewai run --definition flows/research.crewai
```

lib/cli/tests/test_run_declarative_flow.py

@@ -60,6 +60,43 @@ def test_run_declarative_flow_reports_missing_file(
     )
 
 
+def test_run_declarative_flow_reports_empty_file(
+    tmp_path: Path, capsys: pytest.CaptureFixture[str]
+) -> None:
+    definition_path = tmp_path / "flow.yaml"
+    definition_path.write_text(" \n", encoding="utf-8")
+
+    with pytest.raises(SystemExit):
+        run_declarative_flow_module.run_declarative_flow(str(definition_path))
+
+    assert "Flow declaration file is empty" in capsys.readouterr().err
+
+
+@pytest.mark.parametrize(
+    "contents, expected_error",
+    [
+        ("[]\n", "Flow declaration must contain a mapping"),
+        ("schema: crewai.flow/v1\nmethods: {}\n", "Field required"),
+    ],
+)
+def test_load_declarative_flow_reports_invalid_declarations(
+    tmp_path: Path,
+    capsys: pytest.CaptureFixture[str],
+    contents: str,
+    expected_error: str,
+) -> None:
+    definition_path = tmp_path / "flow.yaml"
+    definition_path.write_text(contents, encoding="utf-8")
+
+    with pytest.raises(SystemExit) as exc_info:
+        run_declarative_flow_module.load_declarative_flow(str(definition_path))
+
+    assert exc_info.value.code == 1
+    stderr = capsys.readouterr().err
+    assert f"Unable to read --definition path {definition_path}:" in stderr
+    assert expected_error in stderr
+
+
 def test_run_declarative_flow_in_project_env_uses_uv(
     monkeypatch: pytest.MonkeyPatch, tmp_path: Path
 ) -> None: