fix(providers): dedupe tool_use ids to prevent Anthropic 400s

Anthropic rejects any request where two tool_use blocks share an id
("messages.N.content.M: tool_use ids must be unique"). A mis-assembled
stream could surface the same tool_use block twice in one assistant turn;
the runner persisted it verbatim, so the malformed message was re-sent on
every subsequent turn and permanently bricked the session — the agent
silently stopped replying.

Fix at two layers:
- AnthropicProvider._parse_response: drop duplicate tool_use ids (keep
  first) as the response enters nanobot, so corruption is never persisted.
- AgentRunner._dedup_tool_calls: a new context-governance pass that dedupes
  assistant tool_calls and tool results by id before each send, healing any
  history that was already corrupted.

Add regression tests covering both the dedup and the no-op fast path.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

nanobot/providers/anthropic_provider.py

@@ -10,6 +10,8 @@ import string
 from collections.abc import Awaitable, Callable
 from typing import Any
 
+from loguru import logger
+
 from nanobot.providers.base import (
     LLMProvider,
     LLMResponse,
@@ -522,11 +524,19 @@ class AnthropicProvider(LLMProvider):
         content_parts: list[str] = []
         tool_calls: list[ToolCallRequest] = []
         thinking_blocks: list[dict[str, Any]] = []
+        seen_tool_ids: set[str] = set()
 
         for block in response.content:
             if block.type == "text":
                 content_parts.append(block.text)
             elif block.type == "tool_use":
+                # Anthropic requires every tool_use id to be unique. A mis-assembled
+                # stream can occasionally surface the same block twice; keep the first
+                # and drop the duplicate so it never poisons the persisted history.
+                if block.id in seen_tool_ids:
+                    logger.warning("dropping duplicate tool_use id from response: {}", block.id)
+                    continue
+                seen_tool_ids.add(block.id)
                 tool_calls.append(ToolCallRequest(
                     id=block.id,
                     name=block.name,