fix(ci): strip dart-run noise via byte-level { extraction, not line-based sed

Last run uncovered the actual prefix: `dart run` joins "Running build
hooks..." (sometimes printed multiple times) onto the same line as our
JSON's opening `{`, so my line-based `sed -n '/^{/,$p'` matched no line
and ate the JSON entirely.

Replace it with `json="{${raw#*\{}"` — bash parameter expansion that
extracts from the first `{` byte forward, regardless of newlines. Guard
the empty-output case explicitly so the error message is unambiguous.

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

Author: FeodorFitsner

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

@@ -24,20 +24,25 @@ runs:
           echo "::error::SERIOUS_PYTHON_VERSION is not set in env."
           exit 1
         fi
-        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.
+        flutter pub get >/dev/null
+        # `dart run` writes mixed noise to stdout on a cold cache —
+        # "Building package executable...", "Running build hooks..." — and
+        # the build-hook line gets joined to our JSON's opening `{` with no
+        # newline, so line-based sed patterns like `^{` miss everything.
+        # Strip from the byte-level first `{` forward instead. stderr is
+        # captured alongside stdout so any genuine dart error is visible.
         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 '/^{/,$p')
+        if [[ "$raw" != *"{"* ]]; then
+          echo "::error::dart run produced no JSON for SERIOUS_PYTHON_VERSION=${SERIOUS_PYTHON_VERSION}"
+          printf '%s\n' "$raw"
+          exit 1
+        fi
+        json="{${raw#*\{}"
         full=$(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}"
+          printf '%s\n' "$json"
           exit 1
         fi
         echo "SERIOUS_PYTHON_FULL_VERSION=$full" >> "$GITHUB_ENV"