doc: E2E testing documentation updates

[BYK]

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• If you've added code that should be tested, please add tests.
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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)

[cursor]

Cursor Agent can help with this pull request. Just @cursor in comments and I'll start working on changes in this branch.
_{Learn more about Cursor Agents}

[Lms24]

Thanks!

I'm not the biggest fan of the AI "accent" here, so I had some suggestions to remove the fluff. In general it's valuable content though, so thanks for adding it!

[github-actions]

node-overhead report 🧳

Note: This is a synthetic benchmark with a minimal express app and does not necessarily reflect the real-world performance impact in an application.

Scenario	Requests/s	% of Baseline	Prev. Requests/s	Change %
GET Baseline	9,426	-	9,106	+4%
GET With Sentry	1,741	18%	1,665	+5%
GET With Sentry (error only)	6,179	66%	6,085	+2%
POST Baseline	1,203	-	1,169	+3%
POST With Sentry	570	47%	556	+3%
POST With Sentry (error only)	1,038	86%	1,045	-1%
MYSQL Baseline	3,314	-	3,326	-0%
MYSQL With Sentry	459	14%	435	+6%
MYSQL With Sentry (error only)	2,713	82%	2,691	+1%

View base workflow run