feat: add optional JSON output format for all tools

[CodeKeanu]

Summary

• Added supports_json parameter to the @endpoint decorator that auto-adds a format parameter
• When format="json" is passed, endpoints return structured JSON instead of human-readable text
• Text output remains unchanged (backward compatible)

Changes

• Enhanced @endpoint decorator in base.py to support supports_json=True option
• Auto-adds format parameter with enum ["text", "json"] and default "text"
• Implemented JSON output for 5 key endpoints:
  • get_player_summary: Player profile data with status/visibility enums
  • get_friend_list: Friends array with steam_id and friend_since timestamps
  • get_owned_games: Games array with playtime stats and metadata
  • get_player_achievements: Achievement list with unlock times and completion stats
  • get_app_details: Full app metadata including pricing, platforms, genres

Testing

• All existing tests pass (187 tests)
• Text output unchanged (backward compatible)

Checklist from Issue

• format parameter available on key endpoints
• JSON output is valid and well-structured
• Text output unchanged (backward compatible)
• At least 5 endpoints support JSON mode

Closes #14

🤖 Generated with Claude Code

[CodeKeanu]

Code Review - JSON Output Format

Verdict: ✅ Approve

Strengths:

• Clean decorator extension with supports_json=True
• Backward compatible (default "text")
• Proper dict(params) copy to avoid mutation

Minor Notes (non-blocking):

• format shadows builtin - acceptable for local param
• Inconsistent error JSON: get_owned_games returns {"error": msg, "games": []}, others return just {"error": msg}. Consider standardizing in follow-up.

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Review by @agent-ml-python-mcp-expert