fix(agent): harden tool-call handling against malformed upstream relays

Combine malformed tool-call handling with placeholder filtering and a
no-tools fallback so a relay that returns tool_use blocks with null
id/name/input can no longer crash a turn or permanently wedge a session.

Adapted to the ContextGovernor architecture (context governance now lives
in nanobot/agent/context_governance.py, not runner.py):

- ToolCallRequest.has_valid_name(): single source of truth for "usable
  name" (non-empty string).
- tool_hints.format_tool_hints(): skip tool calls with a non-string/empty
  name instead of raising AttributeError on the whole turn.
- ContextGovernor.strip_placeholder_assistant_messages() and
  strip_malformed_tool_calls() (plus the _tool_call_name_is_valid helper):
  history-cleaning staticmethods invoked at the START of
  prepare_for_model() — strip_placeholder, then strip_malformed, then the
  existing drop_orphan/backfill chain. Both only repair the model-facing
  copy and leave persisted history untouched (return a copy, or the same
  list when nothing changes). Also wired into runner's minimal-repair path.
- AgentRunner._drop_malformed_tool_calls(): returns
  (dropped, all_dropped, original_finish_reason); clears finish_reason to
  "stop" when all calls are dropped.
- AgentRunner._malformed_tool_call_retry_messages() + _request_model
  malformed_retry flag: when an all-dropped tool_calls response comes back,
  retry once with a corrective note; if the retry STILL comes back
  all-dropped, fall back to _request_no_tools for graceful text degradation.

Tests for the history-cleaning methods live with ContextGovernor in
tests/agent/test_runner_governance.py; response-layer and tool-hint tests
stay on AgentRunner / tool_hints.

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

nanobot/providers/base.py

@@ -54,6 +54,18 @@ class ToolCallRequest:
     provider_specific_fields: dict[str, Any] | None = None
     function_provider_specific_fields: dict[str, Any] | None = None
 
+    def has_valid_name(self) -> bool:
+        """Whether this call carries a usable (non-empty string) tool name.
+
+        ToolCallRequest.name is typed ``str`` but not enforced at runtime: a
+        model/gateway can emit a degenerate call with ``name=None`` or ``""``.
+        Such a call cannot be executed and, if persisted and replayed, makes
+        upstream APIs reject the whole request (e.g. Anthropic-style
+        ``messages.content.N.tool_use.name: Input should be a valid string``),
+        which permanently wedges the session.
+        """
+        return isinstance(self.name, str) and bool(self.name)
+
     def to_openai_tool_call(self) -> dict[str, Any]:
         """Serialize to an OpenAI-style tool_call payload."""
         arguments = (