fix(output): fail-closed when AWS credentials missing

[Bartok9]

Summary

Polly synthesis used to proceed even when AWS_ACCESS_KEY_ID / AWS_SECRET_ACCESS_KEY were missing or blank (boto3 then fails noisily, or worse, with partial env). This PR adds a pure aws_credentials_ok helper and fails closed in the feedback callback: log + publish output_error, skip synthesis.

Motivation

Embodied LLM stacks should not call paid cloud TTS without credentials. Matches the fail-closed style already used elsewhere in the series (OpenAI key, etc.).

Changes

• llm_output/llm_output/aws_credentials.py — pure validator
• llm_audio_output.py — early return when invalid
• llm_output/test/test_aws_credentials.py — offline unit tests

Verification

cd llm_output && PYTHONPATH=. python3 test/test_aws_credentials.py -v
# 3 tests OK

AI-assisted; human-reviewed.