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/coding-agent/src/core/agent-session.ts

@@ -680,7 +680,17 @@ export class AgentSession {
 			};
 			const replacement = await this._extensionRunner.emitMessageEnd(extensionEvent);
 			if (replacement) {
-				this._replaceMessageInPlace(event.message, replacement);
+				// Untyped extension handlers can return messages with null/missing content;
+				// normalize so it never enters agent state or session history.
+				const normalized =
+					(replacement.role === "user" ||
+						replacement.role === "assistant" ||
+						replacement.role === "toolResult" ||
+						replacement.role === "custom") &&
+					replacement.content == null
+						? ({ ...replacement, content: [] } as AgentMessage)
+						: replacement;
+				this._replaceMessageInPlace(event.message, normalized);
 			}
 		} else if (event.type === "tool_execution_start") {
 			const extensionEvent: ToolExecutionStartEvent = {
@@ -1143,7 +1153,8 @@ export class AgentSession {
 					messages.push({
 						role: "custom",
 						customType: msg.customType,
-						content: msg.content,
+						// Untyped extensions can pass null/missing content; normalize at ingestion.
+						content: msg.content ?? [],
 						display: msg.display,
 						details: msg.details,
 						timestamp: Date.now(),
@@ -1341,7 +1352,8 @@ export class AgentSession {
 		const appMessage = {
 			role: "custom" as const,
 			customType: message.customType,
-			content: message.content,
+			// Untyped extensions can pass null/missing content; normalize at ingestion.
+			content: message.content ?? [],
 			display: message.display,
 			details: message.details,
 			timestamp: Date.now(),

packages/coding-agent/src/core/session-manager.ts

@@ -378,10 +378,21 @@ function getSessionContextSettings(path: SessionEntry[]): Pick<SessionContext, "
  */
 export function sessionEntryToContextMessages(entry: SessionEntry): AgentMessage[] {
 	if (entry.type === "message") {
-		return [entry.message];
+		const message = entry.message;
+		// Session files are parsed without validation; old versions, forks, or
+		// hand-edited files can contain messages with null/missing content.
+		if (
+			(message.role === "user" || message.role === "assistant" || message.role === "toolResult") &&
+			message.content == null
+		) {
+			return [{ ...message, content: [] }];
+		}
+		return [message];
 	}
 	if (entry.type === "custom_message") {
-		return [createCustomMessage(entry.customType, entry.content, entry.display, entry.details, entry.timestamp)];
+		return [
+			createCustomMessage(entry.customType, entry.content ?? [], entry.display, entry.details, entry.timestamp),
+		];
 	}
 	if (entry.type === "branch_summary" && entry.summary) {
 		return [createBranchSummaryMessage(entry.summary, entry.fromId, entry.timestamp)];