fix: normalize LLM output types in profile serialization

[ygh1254]

Summary

• Add type coercion helpers (_coerce_to_str, _coerce_to_str_list) to safely convert dict/list LLM outputs into plain strings
• Normalize fields at construction time via OasisAgentProfile.__post_init__ — covers bio, persona, country, profession, gender, interested_topics
• Defensive coercion in serializers (_save_twitter_csv, _save_reddit_json) as a second safety net before string operations like [:150] and .replace()
• Normalize in _generate_profile_with_llm() immediately after json.loads() to catch structured outputs before they reach the dataclass

Root Cause

LLM JSON parsing (json.loads) can produce dict or list values for fields declared as str (e.g. bio, persona, country). The serialization code then crashes on string operations like slicing ([:150]) or .replace() applied to non-string types.

Traceback from issue:

File "backend/app/services/oasis_profile_generator.py", line 1296, in _save_reddit_json
  "bio": profile.bio[:150] if profile.bio else f"{profile.name}",
KeyError: slice(None, 150, None)

Testing

The coercion logic handles:

• bio as dict → extracts text via common keys or json.dumps
• persona as dict → same
• country as list → joins into comma-separated string
• interested_topics as nested dicts → flattens to list[str]

Closes #154