feat(tool/cmd/migrate): extract copyright year from existing files

[julieqiu]

Read the copyright year from src/index.ts in each package and set it as copyright_year in the library config. This preserves the original copyright year when libraries are regenerated, since the generator always uses the current year.

For #4413

[gemini-code-assist]

Code Review

The pull request adds functionality to extract the copyright year from existing Node.js package files to preserve it during regeneration. The implementation is straightforward and includes a corresponding test update. I've suggested an improvement to the regular expression to handle year ranges in copyright notices, making the extraction more robust.

[codecov]

Codecov Report

❌ Patch coverage is 88.88889% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 79.40%. Comparing base (1eadeb4) to head (4ccb91f).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
tool/cmd/migrate/nodejs.go	88.88%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4596   +/-   ##
=======================================
  Coverage   79.39%   79.40%           
=======================================
  Files         110      110           
  Lines        9223     9232    +9     
=======================================
+ Hits         7323     7331    +8     
- Misses       1389     1390    +1     
  Partials      511      511           

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