bug: EOF heredoc delimiter collision in GITHUB_OUTPUT causes step failure

TL;DR

The action uses EOF as the heredoc delimiter when writing gemini_response and gemini_errors to $GITHUB_OUTPUT. When Gemini CLI's stderr contains the substring EOF (e.g. from internal bash parser diagnostics like SCRIPT_EOF), it prematurely terminates the heredoc block. The remaining stderr content is then parsed as raw $GITHUB_OUTPUT format, which GitHub Actions rejects with:

##[error]Unable to process file command 'output' successfully.
##[error]Invalid format 'SCRIPT_EOF Syntax Errors: [ 'Error node: "<" at 0:0' ]'

This is related to #381, which reports the same underlying issue from a different angle (JSON output containing EOF leaking into the next output block).

Root Cause

In action.yml, the output blocks use EOF as the delimiter:

echo "gemini_errors<<EOF" >> "${GITHUB_OUTPUT}"
if [[ -n "${ERROR_JSON}" ]]; then
  echo "${ERROR_JSON}" >> "${GITHUB_OUTPUT}"
else
  cat "${TEMP_STDERR}" >> "${GITHUB_OUTPUT}"
fi
echo "EOF" >> "${GITHUB_OUTPUT}"

When TEMP_STDERR contains Gemini CLI's internal bash parser warnings (which include the string SCRIPT_EOF), the EOF at position 7 of SCRIPT_EOF matches the heredoc terminator on a line by itself, breaking the block.

The same issue applies to gemini_response<<EOF when stdout JSON happens to contain EOF on its own line.

Reproduction

This is intermittent — it depends on whether Gemini CLI's bash command parser emits syntax warnings containing EOF during a given run. More likely when the agent writes heredoc scripts (e.g. cat << 'SCRIPT_EOF' > file.py).

Observed on: run-gemini-cli@v0.1.21, still present on latest main (642deeb7).

Suggested Fix

Use a more unique delimiter that won't appear in Gemini CLI output:

echo "gemini_errors<<GEMINI_CLI_OUTPUT_DELIMITER" >> "${GITHUB_OUTPUT}"
...
echo "GEMINI_CLI_OUTPUT_DELIMITER" >> "${GITHUB_OUTPUT}"

Or use a random delimiter per run:

DELIMITER="GEMINI_EOF_$(openssl rand -hex 8)"
echo "gemini_errors<<${DELIMITER}" >> "${GITHUB_OUTPUT}"
...
echo "${DELIMITER}" >> "${GITHUB_OUTPUT}"

Environment

• Action version: v0.1.21
• Runner: ubuntu-latest
• Gemini model: gemini-3.1-pro-preview

Inspect the output-writing blocks in action.yml, focusing on the gemini_response and gemini_errors heredoc delimiters used with GITHUB_OUTPUT. Reproduce with stderr or stdout containing EOF, then verify that the action accepts the outputs without treating embedded Gemini CLI text as raw output format.