fix(ai,agent,coding-agent): normalize null message content at ingestion boundaries (#6343)

The Message types require content to always be present, but untyped JS
extension tools, hand-built histories, and old or hand-edited session
files can violate that contract, crashing rendering, compaction, and
provider request conversion with 'content is not iterable'.

Normalize null/missing content to an empty array at the ingestion
boundaries instead of guarding every consumer:

- transformMessages (choke point before every provider request)
- createToolResultMessage in the agent loop
- session entry loading (message and custom_message entries)
- extension custom messages (sendCustomMessage, before_agent_start)
- message_end extension replacements

fixes #6259, fixes #6276

packages/agent/src/agent-loop.ts

@@ -735,7 +735,9 @@ function createToolResultMessage(finalized: FinalizedToolCallOutcome): ToolResul
 		role: "toolResult",
 		toolCallId: finalized.toolCall.id,
 		toolName: finalized.toolCall.name,
-		content: finalized.result.content,
+		// Untyped tools (JS extensions) can return results without content; normalize
+		// so the null never enters session history or provider payloads.
+		content: finalized.result.content ?? [],
 		details: finalized.result.details,
 		isError: finalized.isError,
 		timestamp: Date.now(),