E2E testing documentation updates

[BYK]

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Closes #issue_link_here

This PR documents key learnings and best practices for E2E testing within the Sentry JavaScript SDKs, focusing on common pitfalls and setup requirements.

Why these changes?
To improve the developer experience for E2E testing by providing clear guidance on Verdaccio setup, the critical .npmrc file, bundler-specific behaviors, and debugging strategies, thereby reducing common issues and debugging time.

What was changed?

• CONTRIBUTING.md: Added a new section "Running E2E Tests Locally" with prerequisites, step-by-step instructions, and troubleshooting.
• CLAUDE.md & .cursor/rules/sdk_development.mdc: Added an "E2E Testing" section under "Development Guidelines," explaining Verdaccio, .npmrc requirements, how to run single tests, and common pitfalls.
• dev-packages/e2e-tests/README.md: Expanded existing documentation with detailed .npmrc explanations, a comprehensive troubleshooting guide, and notes on bundler-specific environment variable handling (Webpack, Vite, Next.js) and import.meta.env behavior.

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Closes #18651 (added automatically)

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Cursor Agent can help with this pull request. Just @cursor in comments and I'll start working on changes in this branch.
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