feat: support request-level proxy sessions

[alexferrari88]

Summary

• Add an optional proxy object to POST /tabs for request-level BrowserContext proxy configuration.
• Keep proxy changes scoped to session creation: the same proxy may be reused for an existing user session, but different proxies return 409.
• Reject request-level proxies when global PROXY_* configuration is active, and document the GeoIP/parity limitation vs the global proxy path.
• Add request proxy validation/redaction helpers, OpenAPI docs, README docs, and unit coverage.
• Fix local Jest wiring so the default npm test suite does not accidentally run e2e files without their global setup, and make npm run test:e2e use the dedicated e2e Jest config.

Why

Some integrations need to choose a proxy per automated task/API request without running separate camofox-browser deployments. Playwright applies proxy settings at BrowserContext creation time, so this implements the narrowest compatible API surface rather than pretending proxy can be changed per navigation.

Test plan

• git diff --check HEAD~1..HEAD
• npm run generate-openapi
• npm test -- tests/unit/requestProxy.test.js tests/unit/openapi.test.js
• npm test -- tests/unit
• npm test
• npm run test:e2e
• Secret scan against local env-derived values: no exact secret matches in repo files.

Notes

• Request-level proxy routes traffic only. It does not currently apply the global proxy path's automatic GeoIP-derived locale/timezone/geolocation behavior.