Fix standalone custom-component execution (streamlit#8620)

## Describe your changes

Closes streamlit#8606

The PR allows custom component modules to be executed standalone by not
registering them with Streamlit at all, which makes sense since
Streamlit is not started.

It's a cherry-pick of this broader PR
streamlit#8610, which also adds tests.
However, we want to discuss our testing strategy first, so we decouple
it from adding the fix (see for example also this PR
streamlit#8616).

## GitHub Issue Link (if applicable)

- streamlit#8606
- NathanChen198/streamlit-rsa-auth-ui#1
- randyzwitch/streamlit-folium#186

## Testing Plan

- E2E Tests: tests to ensure this behavior will come in a separate PR,
see streamlit#8616 and
streamlit#8610

---

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project are made under the Apache 2.0 license.

e2e_playwright/custom_components/popular_components_test.py

@@ -94,6 +94,7 @@ def test_extra_streamlit_components(app: Page):
 def test_folium(app: Page):
     """Test that the folium component renders"""
     _select_component(app, "folium")
+    _expect_no_exception(app)
     _expect_iframe_attached(app)
 
 

lib/streamlit/components/v1/component_registry.py

@@ -21,6 +21,7 @@
 from streamlit.components.types.base_component_registry import BaseComponentRegistry
 from streamlit.components.v1.custom_component import CustomComponent
 from streamlit.runtime import get_instance
+from streamlit.runtime.scriptrunner import get_script_run_ctx
 
 
 def _get_module_name(caller_frame: FrameType) -> str:
@@ -47,7 +48,9 @@ def declare_component(
     path: str | None = None,
     url: str | None = None,
 ) -> CustomComponent:
-    """Create and register a custom component.
+    """Create a custom component and register it if there is a ScriptRun context.
+
+    The component is not registered when there is no ScriptRun context; this can happen when CustomComponents are executed as standalone commands, e.g. for testing.
 
     Parameters
     ----------
@@ -84,8 +87,10 @@ def declare_component(
     component = CustomComponent(
         name=component_name, path=path, url=url, module_name=module_name
     )
-    get_instance().component_registry.register_component(component)
-
+    # the ctx can be None if a custom component script is run outside of Streamlit, e.g. via 'python ...'
+    ctx = get_script_run_ctx()
+    if ctx is not None:
+        get_instance().component_registry.register_component(component)
     return component