fix(precise-date): support TypeScript 7

[hugosmoreira]

Fixes #9031

Summary

Remove the legacy @extends external:Date JSDoc tag from PreciseDate. The TypeScript declaration already expresses the inheritance as extends Date; TypeScript 7 treats the mismatched JSDoc target as error TS8023.

Validation

• Confirmed unmodified main fails under TypeScript 7.0.2 with the reported TS8023 error
• Compiled the package successfully with TypeScript 5.8 and TypeScript 7.0.2 after the change
• Compiled a TypeScript 7.0.2 consumer against the emitted declaration
• Ran all 51 package tests successfully
• Ran the Google TypeScript Style ESLint configuration successfully
• Confirmed code coverage does not decrease
• Updated the affected documentation comment

[gemini-code-assist]

Code Review

This pull request removes the @extends external:Date JSDoc annotation from the class documentation in core/precise-date/src/index.ts. There are no review comments, and I have no feedback to provide.