fix(agent): route finish_reason='length' with blank content to length recovery

When an LLM response arrives with finish_reason='length' and has_tool_calls
but blank text content (e.g. the model spent its whole output budget on a
tool call whose closing tag was truncated), the runner dropped the tool
calls and then misrouted the blank response into the empty-response retry
branch. Retrying the same prompt cannot recover from output-budget
exhaustion, so every retry hit the same length ceiling and the turn ended
in the generic apology.

The length-recovery branch was gated on 'finish_reason == length and not
is_blank_text(clean)', so a blank-but-truncated turn could never reach it.

- The empty-response retry branch now excludes finish_reason == 'length'
  (in addition to 'error').
- The length-recovery branch no longer requires non-blank content, so a
  blank-but-truncated turn enters recovery and appends
  build_length_recovery_message (which handles a blank tail safely).

Adds a regression test asserting the length-recovery path is taken; it
fails on the unfixed code and passes with the fix.

Fixes #5133

nanobot/agent/runner.py

@@ -575,7 +575,7 @@ class AgentRunner:
                 )
 
             clean = hook.finalize_content(context, response.content)
-            if response.finish_reason != "error" and is_blank_text(clean):
+            if response.finish_reason not in ("error", "length") and is_blank_text(clean):
                 empty_content_retries += 1
                 if empty_content_retries < _MAX_EMPTY_RETRIES:
                     logger.warning(
@@ -608,7 +608,7 @@ class AgentRunner:
                 original_content = response.content
                 clean = hook.finalize_content(context, response.content)
 
-            if response.finish_reason == "length" and not is_blank_text(clean):
+            if response.finish_reason == "length":
                 if len(length_recovery_parts) < _MAX_LENGTH_RECOVERIES:
                     length_recovery_parts.append(
                         _restore_outer_whitespace(clean or "", original_content)