feat: structured-output Trader and Research Manager (#434, finishes the trio)

Extends the canonical structured-output pattern from the Portfolio Manager
to the other two decision-making agents.  Each of the three agents now
returns a typed Pydantic instance via llm.with_structured_output() in a
single primary call, and a render helper turns the result into the same
markdown shape downstream agents and saved reports already consume.

- ResearchPlan: 5-tier recommendation, conversational rationale, concrete
  strategic actions for the trader.
- TraderProposal: 3-tier action (transaction direction is naturally Buy /
  Hold / Sell — position sizing happens later at the Portfolio Manager),
  reasoning, and optional entry_price / stop_loss / position_sizing.
  Rendered output preserves the trailing "FINAL TRANSACTION PROPOSAL:
  **BUY/HOLD/SELL**" line for backward compatibility with the analyst
  stop-signal text.
- PortfolioDecision: 5-tier rating, executive summary, investment thesis,
  optional price_target / time_horizon (unchanged).

The shared try-structured-then-fallback pattern is extracted into
tradingagents/agents/utils/structured.py (bind_structured +
invoke_structured_or_freetext) so all three agents go through the same
code path and log the same warning when a provider lacks structured
output and the agent falls back to free-text generation.

Net effect for users: every saved markdown report (research/manager.md,
trading/trader.md, portfolio/decision.md) now has consistent section
headers across runs and providers, easier to scan.

Net effect for the runtime: the rating extraction round-trip is gone —
the rating comes from the structured response itself, not a second
LLM call. SignalProcessor was already simplified to a heuristic adapter
in the previous commit.

11 new tests in tests/test_structured_agents.py cover the Trader and
Research Manager render functions, structured-output happy paths, and
free-text fallback. Full suite: 88 tests pass in ~2s without API keys.

tradingagents/agents/utils/structured.py

@@ -0,0 +1,73 @@
+"""Shared helpers for invoking an agent with structured output and a graceful fallback.
+
+The Portfolio Manager, Trader, and Research Manager all follow the same
+canonical pattern:
+
+1. At agent creation, wrap the LLM with ``with_structured_output(Schema)``
+   so the model returns a typed Pydantic instance. If the provider does
+   not support structured output (rare; mostly older Ollama models), the
+   wrap is skipped and the agent uses free-text generation instead.
+2. At invocation, run the structured call and render the result back to
+   markdown. If the structured call itself fails for any reason
+   (malformed JSON from a weak model, transient provider issue), fall
+   back to a plain ``llm.invoke`` so the pipeline never blocks.
+
+Centralising the pattern here keeps the agent factories small and ensures
+all three agents log the same warnings when fallback fires.
+"""
+
+from __future__ import annotations
+
+import logging
+from typing import Any, Callable, Optional, TypeVar
+
+from pydantic import BaseModel
+
+logger = logging.getLogger(__name__)
+
+T = TypeVar("T", bound=BaseModel)
+
+
+def bind_structured(llm: Any, schema: type[T], agent_name: str) -> Optional[Any]:
+    """Return ``llm.with_structured_output(schema)`` or ``None`` if unsupported.
+
+    Logs a warning when the binding fails so the user understands the agent
+    will use free-text generation for every call instead of one-shot fallback.
+    """
+    try:
+        return llm.with_structured_output(schema)
+    except (NotImplementedError, AttributeError) as exc:
+        logger.warning(
+            "%s: provider does not support with_structured_output (%s); "
+            "falling back to free-text generation",
+            agent_name, exc,
+        )
+        return None
+
+
+def invoke_structured_or_freetext(
+    structured_llm: Optional[Any],
+    plain_llm: Any,
+    prompt: Any,
+    render: Callable[[T], str],
+    agent_name: str,
+) -> str:
+    """Run the structured call and render to markdown; fall back to free-text on any failure.
+
+    ``prompt`` is whatever the underlying LLM accepts (a string for chat
+    invocations, a list of message dicts for chat models that take that
+    shape). The same value is forwarded to the free-text path so the
+    fallback sees the same input the structured call did.
+    """
+    if structured_llm is not None:
+        try:
+            result = structured_llm.invoke(prompt)
+            return render(result)
+        except Exception as exc:
+            logger.warning(
+                "%s: structured-output invocation failed (%s); retrying once as free text",
+                agent_name, exc,
+            )
+
+    response = plain_llm.invoke(prompt)
+    return response.content