chore: update .nycrc, Contributing.md, and .gitignore across packages

[bshaffer]

Separated from #8973 to keep the core PNPM migration PR focused.

This PR applies the changes across subpackages for:

• .nycrc configuration updates
• CONTRIBUTING.md instructions (updating npm -> pnpm)
• .gitignore cleanups across packages

[gemini-code-assist]

Code Review

This pull request migrates package management and scripts from npm to pnpm across multiple packages, updating CONTRIBUTING.md files, removing package-lock.json and yarn.lock from .gitignore files, and simplifying coverage reporters in .nycrc files. Feedback highlights that some instances of npm run samples-test were missed in the CONTRIBUTING.md updates, and warns against manually editing auto-generated baseline and test-fixture files instead of updating the upstream generator templates.

[gemini-code-assist]

In this file (and many other CONTRIBUTING.md files across the repository), the command npm run samples-test on line 61 was missed during the migration from npm to pnpm. Please update it to pnpm run samples-test for consistency.

[gemini-code-assist]

It appears that CONTRIBUTING.md (and potentially .gitignore files) in the baseline and test-fixture directories are being manually updated here. According to the repository rules, manual edits to auto-generated files or baseline files should be avoided as they will be overwritten during the next regeneration. Please ensure that the upstream generator templates (or the source files used by the generator) are updated instead, and then regenerate these baseline/test-fixture files.

References

1. Do not manually edit auto-generated files to fix typos or make other changes, as these edits will be overwritten during the next regeneration. Instead, apply the fixes upstream in the generator or templates.