fix: filter empty error messages to prevent session corruption

When 429/500 errors occur during tool execution, empty assistant messages
with stopReason='error' and content=[] get persisted to the session file.

These break the tool_use -> tool_result chain that Claude/Gemini require:
- Claude expects every tool_use block to have a matching tool_result
- Empty error messages inserted mid-sequence violate this invariant
- Results in: 'tool_use ids were found without tool_result blocks'

This patch filters out empty error messages when building session context,
allowing sessions to recover gracefully from transient API errors.

Evidence from production:
- 113 of 170 sessions had empty error messages
- Session 30764430 demonstrated recovery: 429 at 14:30:11 IST,
  resumed successfully at 14:30:22, completed at 14:30:34

Sorry Peter, one more patch! 🙈

package.json

@@ -170,7 +170,8 @@
       "@sinclair/typebox": "0.34.47"
     },
     "patchedDependencies": {
-      "@mariozechner/pi-agent-core": "patches/@mariozechner__pi-agent-core.patch"
+      "@mariozechner/pi-agent-core": "patches/@mariozechner__pi-agent-core.patch",
+      "@mariozechner/pi-coding-agent": "patches/@mariozechner__pi-coding-agent.patch"
     }
   },
   "vitest": {