Isolate malformed LLM batch responses

[koriyoshi2041]

Summary

• skip only the malformed LLM batch when structured response parsing fails
• keep configuration errors such as missing API keys propagating
• add sync run_batches regression coverage for a stray-text structured response

Fixes #250.

Verification

• uv run pytest tests/nodes/test_llm_analyzer_base.py -k "RunBatches or failed_batch or value_error"
• uv run pytest tests/nodes/test_llm_analyzer_base.py
• git diff --check upstream/main...HEAD
• uv run ruff check src/ tests/
• uv run ruff format --check src/ tests/

Note: make lint and make format fail in this shell because the Makefile calls bare ruff, but the equivalent uv run ruff ... commands above pass.