fix(client-certificates): respect launch proxy and proxy.bypass

[Yury Semikhatsky (yury-s)]

Summary

• The client certificates interceptor now falls back to the launch-level proxy when the context has no proxy.
• proxy.bypass is applied to intercepted connections.

Fixes #42806

[github-actions]

Test results for "tests 1"

8 flaky

⚠️ [chromium-library] › library/chromium/connect-to-worker.spec.ts:19 › should connect, evaluate, receive console and disconnect `@frozen-time-library-chromium-linux`
⚠️ [chromium-library] › library/video.spec.ts:690 › screencast › should capture full viewport `@chromium-ubuntu-22.04-node24`
⚠️ [chromium-library] › library/popup.spec.ts:260 › should not throw when click closes popup `@chromium-ubuntu-22.04-node22`
⚠️ [chromium-library] › library/video.spec.ts:762 › screencast › should work with video+trace `@chromium-ubuntu-22.04-node22`
⚠️ [firefox-library] › library/browsercontext-cookies-third-party.spec.ts:257 › third party 'Partitioned;' cookies `@firefox-ubuntu-22.04-node20`
⚠️ [firefox-library] › library/browsercontext-cookies-third-party.spec.ts:470 › top level 'Partitioned;' cookie and same origin iframe `@firefox-ubuntu-22.04-node20`
⚠️ [webkit-page] › page/page-leaks.spec.ts:136 › expect should not leak `@webkit-ubuntu-22.04-node20`
⚠️ [playwright-test] › ui-mode-test-output.spec.ts:80 › should show console messages for test `@ubuntu-latest-node26`

52156 passed, 1253 skipped

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Merge workflow run.

[github-actions]

Test results for "MCP"

1 failed
❌ [chromium] › mcp/cli-webmcp.spec.ts:135 › webmcp-list stitches tools across frames @mcp-macos-latest-chromium

8691 passed, 1474 skipped

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Merge workflow run.

[github-actions]

🟢 Nothing here looks caused by this PR

The single failure is a known macOS flake in the WebMCP tool list, and this PR only touches the client-certificates proxy path.

Details

One failed test across both reports; the other 8 entries are flaky (rescued on retry), so nothing to triage there.

Pre-existing flake / infra

• [chromium] › mcp/cli-webmcp.spec.ts:135 › webmcp-list stitches tools across frames (mcp-macos-latest-chromium) — the frame with the second tool isn't registered yet when webmcp-list runs, so it reports Found 1 WebMCP tool(s) instead of 2. In the aggregated CI results this test has failed 4 of 84 runs on mcp-macos-latest-chromium and passed the other 80, and it has never failed on the ubuntu or windows MCP bots. The other three failures carry the byte-identical error on unrelated branches — #42785 (fabe18a1, d29eeae5) and #42792 (67318a99) — so this PR can't be responsible.

This PR changes socksClientCertificatesInterceptor.ts, browser.ts and browserContext.ts to fall back to the launch-level proxy and honour proxy.bypass. The WebMCP path doesn't use client certificates or a proxy, and the assertion that fails is about cross-frame tool registration timing, not networking.

_{Triaged by the Playwright bot - agent run}