feat(cdp-proxy): add /json endpoint for Playwright connectOverCDP support

[rgarcia]

Summary

The CDP proxy on port 9222 previously only implemented /json/version, which returned the WebSocket URL but didn't support target discovery.

Playwright's connectOverCDP() fetches /json to discover browser targets before establishing a WebSocket connection. Without this endpoint, using http://127.0.0.1:9222 with agent-browser or Playwright would fail, even though direct WebSocket connections (ws://127.0.0.1:9222) worked fine.

Changes

• Adds /json and /json/list endpoints that proxy to Chrome's /json
• Rewrites webSocketDebuggerUrl and devtoolsFrontendUrl in the response to use the proxy's host instead of Chrome's internal host
• Enables agent-browser --cdp http://127.0.0.1:9222 to work correctly

Before

# This worked (direct WebSocket):
agent-browser --cdp ws://127.0.0.1:9222 snapshot  # ✓

# This failed (HTTP with target discovery):
agent-browser --cdp http://127.0.0.1:9222 snapshot  # ✗

After

# Both work now:
agent-browser --cdp ws://127.0.0.1:9222 snapshot   # ✓
agent-browser --cdp http://127.0.0.1:9222 snapshot # ✓

Test plan

• Run existing devtoolsproxy tests: go test ./lib/devtoolsproxy/...
• Deploy to a test VM and verify curl http://127.0.0.1:9222/json returns targets with rewritten URLs
• Test agent-browser --cdp http://127.0.0.1:9222 snapshot works inside a Kernel browser VM

---

Note

Medium Risk
Introduces new HTTP handlers that fetch and transform upstream browser JSON; failures could break CDP client connectivity or add latency, but changes are isolated to the devtools proxy surface.

Overview
Adds HTTP target-discovery support to the DevTools proxy on :9222 by implementing /json and /json/list (plus trailing-slash variants) that proxy Chrome’s /json response and rewrite webSocketDebuggerUrl/devtoolsFrontendUrl hosts back to the proxy via rewriteWSURL.

Extends /json/version handling to also accept a trailing slash to avoid Playwright 426s, adds new e2e coverage validating the rewritten endpoints and a real agent-browser flow through the proxy, and hardens the devtoolsproxy Chromium restart test by using stdbuf to reduce CI flakiness when waiting for the “DevTools listening” log line.

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