Address issues in core package

[Mirza-Samad-Ahmed-Baig]

This commit addresses 10 critical and minor code-level issues found in the events.ts and types.ts files within the ag-ui/typescript-sdk/packages/core package.
These fixes improve type safety, consistency, and maintainability of the core functionalities.

Detailed Changes:

events.ts:

1. Removed redundant schema definitions: Eliminated duplicate definitions for RunStartedSchema, RunFinishedSchema, RunErrorSchema, StepStartedSchema, and
   StepFinishedSchema. This reduces redundancy and potential for inconsistencies.
2. Fixed ToolCallResultEventSchema extension: Ensured ToolCallResultEventSchema extends BaseEventSchema. This brings consistency in event structure, ensuring
   all events inherit common properties like timestamp and rawEvent.
3. Made messageId and role required in TextMessageChunkEventSchema: Changed messageId and role from optional to required. This improves data integrity by
   ensuring each text message chunk is properly attributed.
4. Redefined ThinkingTextMessageContentEventSchema: Simplified its definition by directly extending BaseEventSchema and including the delta field. This makes
   the schema definition clearer and more consistent with other event schemas.
5. Clarified StateDeltaEventSchema's delta: Added a more descriptive comment for the delta field, explicitly stating it should conform to JSON Patch (RFC
   6902). This provides better guidance on the expected data structure.

types.ts:
6. ToolMessageSchema extends BaseMessageSchema: Modified ToolMessageSchema to extend BaseMessageSchema. This eliminates code duplication and ensures
ToolMessage inherits common message properties.
7. Clarified ToolSchema's parameters: Added a comment to ToolSchema indicating that parameters should be a JSON Schema object. This provides better context
for developers using this schema.
8. Clarified RunAgentInputSchema's state and forwardedProps: Added comments to RunAgentInputSchema to clarify the arbitrary nature of state and
forwardedProps. This helps in understanding the expected data types for these flexible fields.
9. Clarified StateSchema: Added a comment to StateSchema to indicate it represents an arbitrary state object. This clarifies its permissive nature.
10. Enhanced AGUIError: Added an optional code property to the AGUIError class. This allows for more specific error categorization and handling, improving
debugging and error management within the application.