docs(puppeteer): document embedded browser background network traffic

[AbhiPra24]

Description

Documents embedded Chromium browser background network traffic in src/puppeteer/README.md.

Server Details

• Server: puppeteer
• Changes to: docs (README.md)

Motivation and Context

Fixes #4829.

Browsers like Chromium automatically generate background network traffic (such as component updater, CRL sets, and captive portal / connectivity checks to Google endpoints like clients2.google.com and accounts.google.com) in addition to user-initiated navigation requests. In egress-restricted or audited enterprise environments, documenting these endpoints and proxy considerations prevents confusion during network reviews or auditing.

How Has This Been Tested?

Verified markdown formatting and rendered output of src/puppeteer/README.md.

Breaking Changes

None.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update

Checklist

• I have read the MCP Protocol Documentation
• My changes follows MCP security best practices
• I have updated the server's README accordingly
• I have tested this with an LLM client
• My code follows the repository's style guidelines
• New and existing tests pass locally
• I have added appropriate error handling
• I have documented all environment variables and configuration options

Additional context

Addresses issue #4829.