fix(tools): deduplicate tool names at API boundary for Vertex/Azure/Bedrock

Providers like Google Vertex, Azure, and Amazon Bedrock reject API
requests with duplicate tool names (HTTP 400: 'Tool names must be
unique').  The upstream injection paths in run_agent.py already dedup
after PR #17335, but two API-boundary functions pass tools through
without checking:

- agent/auxiliary_client.py: _build_call_kwargs() (all non-Anthropic
  providers in chat_completions mode)
- agent/anthropic_adapter.py: convert_tools_to_anthropic() (Anthropic
  Messages API path)

Add defensive dedup guards at both sites.  Duplicates are dropped with
a warning log, converting a hard 400 failure into a recoverable
condition.  This is intentionally conservative — the root-cause dedup
in run_agent.py is the primary defense; these guards add resilience
against future injection-path regressions.

Includes 8 new tests covering unique passthrough, duplicate removal,
empty/None edge cases.

Closes #18478

agent/anthropic_adapter.py

@@ -1241,10 +1241,24 @@ def convert_tools_to_anthropic(tools: List[Dict]) -> List[Dict]:
     if not tools:
         return []
     result = []
+    seen_names: set = set()
     for t in tools:
         fn = t.get("function", {})
+        name = fn.get("name", "")
+        # Defensive dedup: Anthropic rejects requests with duplicate tool
+        # names.  Upstream injection paths already dedup, but this guard
+        # converts a hard API failure into a warning.  See: #18478
+        if name and name in seen_names:
+            logger.warning(
+                "convert_tools_to_anthropic: duplicate tool name '%s' "
+                "— dropping second occurrence",
+                name,
+            )
+            continue
+        if name:
+            seen_names.add(name)
         result.append({
-            "name": fn.get("name", ""),
+            "name": name,
             "description": fn.get("description", ""),
             "input_schema": _normalize_tool_input_schema(
                 fn.get("parameters", {"type": "object", "properties": {}})