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! 🙈

pnpm-lock.yaml

@@ -11,6 +11,9 @@ patchedDependencies:
   '@mariozechner/pi-agent-core':
     hash: 01312ceb1f6be7e42822c24c9a7a4f7db56b24ae114a364855bd3819779d1cf4
     path: patches/@mariozechner__pi-agent-core.patch
+  '@mariozechner/pi-coding-agent':
+    hash: 58af7c712ebe270527c2ad9d3351fac39d6cd4b81cc475a258d87840b446b90e
+    path: patches/@mariozechner__pi-coding-agent.patch
 
 importers:
 
@@ -39,7 +42,7 @@ importers:
         version: 0.41.0(ws@8.19.0)(zod@4.3.5)
       '@mariozechner/pi-coding-agent':
         specifier: ^0.41.0
-        version: 0.41.0(ws@8.19.0)(zod@4.3.5)
+        version: 0.41.0(patch_hash=58af7c712ebe270527c2ad9d3351fac39d6cd4b81cc475a258d87840b446b90e)(ws@8.19.0)(zod@4.3.5)
       '@mariozechner/pi-tui':
         specifier: ^0.41.0
         version: 0.41.0
@@ -3807,7 +3810,7 @@ snapshots:
       - ws
       - zod
 
-  '@mariozechner/pi-coding-agent@0.41.0(ws@8.19.0)(zod@4.3.5)':
+  '@mariozechner/pi-coding-agent@0.41.0(patch_hash=58af7c712ebe270527c2ad9d3351fac39d6cd4b81cc475a258d87840b446b90e)(ws@8.19.0)(zod@4.3.5)':
     dependencies:
       '@mariozechner/clipboard': 0.3.0
       '@mariozechner/pi-agent-core': 0.41.0(patch_hash=01312ceb1f6be7e42822c24c9a7a4f7db56b24ae114a364855bd3819779d1cf4)(ws@8.19.0)(zod@4.3.5)