feat: add set_extra_http_headers tool for Network.setExtraHTTPHeaders

[JayeeHsu]

Summary

• Add a new MCP tool set_extra_http_headers that allows setting extra HTTP headers on all page requests via Puppeteer's page.setExtraHTTPHeaders()
• Headers persist across navigations and can be cleared by passing an empty object
• This addresses the need for injecting custom headers (e.g. swim-lane routing, A/B testing, feature flags) on all request types including initial document loads

Closes #1175

Test plan

• Test setting multiple headers returns correct count and header names
• Test clearing headers with empty object returns clear confirmation
• Test headers are actually included in subsequent requests (end-to-end via echo server)

🤖 Generated with Claude Code

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[JayeeHsu]

I have signed the CLA.

[JayeeHsu]

@wolfib Could you help review this?

[OrKoN]

Thanks for the PR! Please align the implementation with the comment in #1175 (comment) To also work better with the CLI interface, perhaps we can have a string format for expressing headers.