fix createTool type with exactOptionalPropertyTypes (#12325)

## Description

Fixed type error when using `createTool` with Agent when
`exactOptionalPropertyTypes: true` is enabled in TypeScript config.

The `ProviderDefinedTool` structural type in `@internal/external-types`
had two incompatibilities with `ToolAction`:
1. `inputSchema` was required but `ToolAction.inputSchema` is optional
2. `execute?` didn't allow explicit `undefined` values, which
`exactOptionalPropertyTypes` distinguishes from absent properties

Changes:
- `inputSchema: unknown` → `inputSchema?: unknown`
- `execute?: (...args: any[]) => any` → `execute?: ((...args: any[]) =>
any) | undefined`

These changes make the types more permissive for users with stricter
TypeScript configs while having no effect for users without
`exactOptionalPropertyTypes`.

## Related Issue(s)

Fixes #12281

## Type of Change

- [x] Bug fix (non-breaking change that fixes an issue)
- [ ] New feature (non-breaking change that adds functionality)
- [ ] Breaking change (fix or feature that would cause existing
functionality to change)
- [ ] Documentation update
- [ ] Code refactoring
- [ ] Performance improvement
- [ ] Test update

## Checklist

- [x] I have made corresponding changes to the documentation (if
applicable)
- [ ] I have added tests that prove my fix is effective or that my
feature works

<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->

## Summary by CodeRabbit

* **Bug Fixes**
* Fixed type compatibility for `createTool` with Agent configurations,
correcting optional property handling to support strict TypeScript type
checking.

<sub>✏️ Tip: You can customize this high-level summary in your review
settings.</sub>

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

Author: graysonhicks

.changeset/good-falcons-win.md

@@ -0,0 +1,5 @@
+---
+'@mastra/core': patch
+---
+
+Fixed type error when using createTool with Agent when exactOptionalPropertyTypes is enabled in TypeScript config. The ProviderDefinedTool structural type now correctly marks inputSchema as optional and allows execute to be undefined, matching the ToolAction interface. Fixes #12281

packages/_external-types/src/index.ts

@@ -25,18 +25,18 @@ export type ProviderDefinedTool =
       type?: string;
       id?: string;
       args?: Record<string, unknown>;
-      execute?: (...args: any[]) => any;
+      execute?: ((...args: any[]) => any) | undefined;
       [key: string]: any; // Allows experimental_* and other future properties
     }
   | {
       // ToolV5 structure
-      inputSchema: unknown;
+      inputSchema?: unknown;
       description?: string;
       type?: string;
       id?: string;
       name?: string;
       providerOptions?: any;
-      execute?: (...args: any[]) => any;
+      execute?: ((...args: any[]) => any) | undefined;
       outputSchema?: any;
       [key: string]: any; // Allows onInput* callbacks and other future properties
     };