feat(agent,coding-agent): add prepareArguments hook for pre-validation argument preparation

Add AgentTool.prepareArguments and ToolDefinition.prepareArguments hook
that runs before schema validation in the agent loop. This lets tools
silently accept legacy argument shapes from resumed old sessions without
polluting the public schema.

The built-in edit tool uses this to fold legacy top-level oldText/newText
into edits[] when resuming sessions that predate the edits-only schema.

- AgentTool/ToolDefinition: typed prepareArguments returning Static<TParameters>
- agent-loop: prepareToolCallArguments() runs before validateToolArguments()
- edit tool: prepareEditArguments folds legacy fields, validateEditInput is strict
- Documented in extensions.md with edit-tool example

packages/coding-agent/docs/extensions.md

@@ -1008,6 +1008,12 @@ pi.registerTool({
     action: StringEnum(["list", "add"] as const),
     text: Type.Optional(Type.String()),
   }),
+  prepareArguments(args) {
+    // Optional compatibility shim. Runs before schema validation.
+    // Return the current schema shape, for example to fold legacy fields
+    // into the modern parameter object.
+    return args;
+  },
 
   async execute(toolCallId, params, signal, onUpdate, ctx) {
     // Stream progress
@@ -1472,6 +1478,14 @@ pi.registerTool({
     action: StringEnum(["list", "add"] as const),  // Use StringEnum for Google compatibility
     text: Type.Optional(Type.String()),
   }),
+  prepareArguments(args) {
+    if (!args || typeof args !== "object") return args;
+    const input = args as { action?: string; oldAction?: string };
+    if (typeof input.oldAction === "string" && input.action === undefined) {
+      return { ...input, action: input.oldAction };
+    }
+    return args;
+  },
 
   async execute(toolCallId, params, signal, onUpdate, ctx) {
     // Check for cancellation
@@ -1515,6 +1529,53 @@ async execute(toolCallId, params) {
 
 **Important:** Use `StringEnum` from `@mariozechner/pi-ai` for string enums. `Type.Union`/`Type.Literal` doesn't work with Google's API.
 
+**Argument preparation:** `prepareArguments(args)` is optional. If defined, it runs before schema validation and before `execute()`. Use it to mimic an older accepted input shape when pi resumes an older session whose stored tool call arguments no longer match the current schema. Return the object you want validated against `parameters`. Keep the public schema strict. Do not add deprecated compatibility fields to `parameters` just to keep old resumed sessions working.
+
+Example: an older session may contain an `edit` tool call with top-level `oldText` and `newText`, while the current schema only accepts `edits: [{ oldText, newText }]`.
+
+```typescript
+pi.registerTool({
+  name: "edit",
+  label: "Edit",
+  description: "Edit a single file using exact text replacement",
+  parameters: Type.Object({
+    path: Type.String(),
+    edits: Type.Array(
+      Type.Object({
+        oldText: Type.String(),
+        newText: Type.String(),
+      }),
+    ),
+  }),
+  prepareArguments(args) {
+    if (!args || typeof args !== "object") return args;
+
+    const input = args as {
+      path?: string;
+      edits?: Array<{ oldText: string; newText: string }>;
+      oldText?: unknown;
+      newText?: unknown;
+    };
+
+    if (typeof input.oldText !== "string" || typeof input.newText !== "string") {
+      return args;
+    }
+
+    return {
+      ...input,
+      edits: [...(input.edits ?? []), { oldText: input.oldText, newText: input.newText }],
+    };
+  },
+  async execute(toolCallId, params, signal, onUpdate, ctx) {
+    // params now matches the current schema
+    return {
+      content: [{ type: "text", text: `Applying ${params.edits.length} edit block(s)` }],
+      details: {},
+    };
+  },
+});
+```
+
 ### Overriding Built-in Tools
 
 Extensions can override built-in tools (`read`, `bash`, `edit`, `write`, `grep`, `find`, `ls`) by registering a tool with the same name. Interactive mode displays a warning when this happens.