[codex] Harden MiniMax content generation fallbacks

[g5n-dev]

What changed

• hardened MiniMax response parsing so blocks with text but no explicit type are still treated as text
• enabled structural fallback for metadata and tag_intro requests when MiniMax returns thinking-only responses
• changed post generation to drop guard-failed sections and keep publishing when the remaining body is still valid
• added regression coverage for MiniMax compatibility parsing and guard-based post degradation

Why it changed

The content pipeline was failing in CI when MiniMax returned thinking-only or malformed compatibility responses. That surfaced as tagging failures, generation retries, and eventually 0 Markdown posts with a fatal output-guard error.

Root cause

• the compatibility parser assumed text blocks always had a type="text"
• metadata/tagging requests did not get the same fallback path as generation/classification requests
• guard failures on a few sections caused the entire post to be skipped even when the rest of the article was still publishable

Impact

• tagging and metadata calls are more resilient to MiniMax compatibility quirks
• content generation no longer fails the whole run just because engaging_intro or deep_comment is unusable
• invalid guarded sections are removed instead of being published

Validation

python3 -m pytest tests/test_ai_filter_fallback.py tests/test_anthropic_response_parsing.py tests/test_generate_content_guards.py tests/test_generator_guard_repair.py tests/test_markdown_normalizer.py tests/test_runtime_profiles.py tests/test_search_fallback.py tests/test_tag_graph_runtime.py -q

Result: 42 passed in 1.94s