chore: release v0.2.4 — structured agents, checkpoint, memory log, providers

This release bundles substantial work since v0.2.3:

- Structured-output Research Manager, Trader, and Portfolio Manager
  (canonical with_structured_output pattern, single LLM call per agent,
  rendered markdown preserves the existing report shape).
- LangGraph checkpoint resume for crash recovery (--checkpoint flag).
- Persistent decision log replacing the per-agent BM25 memory, with
  deferred reflection driven by yfinance returns + alpha vs SPY.
- DeepSeek, Qwen, GLM, and Azure OpenAI provider support; dynamic
  OpenRouter model selection.
- Docker support; cache and logs moved to ~/.tradingagents/ to fix
  Docker permission issues.
- Windows UTF-8 encoding fix on every file I/O site.
- 5-tier rating consistency (Buy / Overweight / Hold / Underweight / Sell)
  across Research Manager, Portfolio Manager, signal processor, memory log.

Plus the small quality items in this commit:

1. Suppress noisy Pydantic serializer warnings from OpenAI Responses-API
   parse path by defaulting structured-output to method="function_calling"
   (root-cause fix, not a warnings filter — same typed result, no warnings).
2. Ship scripts/smoke_structured_output.py so contributors can verify
   their provider's structured-output path with one command.
3. Add opt-in memory_log_max_entries config — when set, oldest resolved
   memory log entries are pruned once the cap is exceeded; pending
   entries (unresolved) are never pruned.
4. backend_url default changed from the OpenAI URL to None so the
   per-provider client falls back to its native endpoint instead of
   leaking OpenAI's URL into Gemini / other clients.

CHANGELOG.md added with the full v0.2.4 entry. 92 tests pass without API keys.

tradingagents/default_config.py

@@ -7,6 +7,10 @@ DEFAULT_CONFIG = {
     "results_dir": os.getenv("TRADINGAGENTS_RESULTS_DIR", os.path.join(_TRADINGAGENTS_HOME, "logs")),
     "data_cache_dir": os.getenv("TRADINGAGENTS_CACHE_DIR", os.path.join(_TRADINGAGENTS_HOME, "cache")),
     "memory_log_path": os.getenv("TRADINGAGENTS_MEMORY_LOG_PATH", os.path.join(_TRADINGAGENTS_HOME, "memory", "trading_memory.md")),
+    # Optional cap on the number of resolved memory log entries. When set,
+    # the oldest resolved entries are pruned once this limit is exceeded.
+    # Pending entries are never pruned. None disables rotation entirely.
+    "memory_log_max_entries": None,
     # LLM settings
     "llm_provider": "openai",
     "deep_think_llm": "gpt-5.4",