ci(resolve-python-vars): surface dart run stdout+stderr for diagnosis

Last CI run failed with "did not return values for
SERIOUS_PYTHON_VERSION=3.12" but the composite action's debug `echo
"$json"` printed nothing — couldn't tell from the log whether `dart
run` errored silently or returned non-JSON. Capture stdout+stderr into
a single var, print it verbatim before piping through sed/jq, and let
`flutter pub get` print its own progress (was previously >/dev/null).

Once we see the actual output we can decide if this is a Dart precompile
hiccup, a package_config issue, or something else.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: FeodorFitsner

.github/actions/resolve-python-vars/action.yml

@@ -24,16 +24,20 @@ runs:
           echo "::error::SERIOUS_PYTHON_VERSION is not set in env."
           exit 1
         fi
-        flutter pub get >/dev/null
-        # `dart run` writes "Building package executable..." to stdout on a
-        # cold cache, ahead of our JSON. Strip everything before the opening
-        # `{` so `jq` sees only the document.
-        json=$(dart run serious_python:main version --json | sed -n '/^{/,$p')
-        full=$(echo "$json" | jq -r ".python_releases.\"${SERIOUS_PYTHON_VERSION}\".standalone_version")
-        date=$(echo "$json" | jq -r ".python_releases.\"${SERIOUS_PYTHON_VERSION}\".python_build_release_date")
+        flutter pub get
+        # Capture stdout+stderr of `dart run` so any failure prose is visible
+        # for diagnosis. `dart run` writes "Building package executable..." to
+        # stdout on a cold cache ahead of our JSON; strip everything before
+        # the opening `{` so `jq` sees only the document.
+        raw=$(dart run serious_python:main version --json 2>&1)
+        printf '%s\n' "--- raw output from dart run ---"
+        printf '%s\n' "$raw"
+        printf '%s\n' "--- end raw output ---"
+        json=$(printf '%s\n' "$raw" | sed -n '/^{/,$p')
+        full=$(printf '%s\n' "$json" | jq -r ".python_releases.\"${SERIOUS_PYTHON_VERSION}\".standalone_version")
+        date=$(printf '%s\n' "$json" | jq -r ".python_releases.\"${SERIOUS_PYTHON_VERSION}\".python_build_release_date")
         if [[ -z "$full" || "$full" == "null" || -z "$date" || "$date" == "null" ]]; then
           echo "::error::serious_python:main version --json did not return values for SERIOUS_PYTHON_VERSION=${SERIOUS_PYTHON_VERSION}"
-          echo "$json"
           exit 1
         fi
         echo "SERIOUS_PYTHON_FULL_VERSION=$full" >> "$GITHUB_ENV"