fix(proxy): allow concurrent proxy dispatcher streams

[wilsonicdev]

Summary

Fixes #4163 by allowing cached HTTP/SOCKS proxy dispatchers to open multiple concurrent upstream sockets. With pipelining: 0, long-lived SSE streams such as Codex /v1/responses can otherwise serialize through a single proxy connection when several requests share the same account-level proxy.

Changes:

• adds OMNIROUTE_PROXY_DISPATCHER_CONNECTIONS with default 32 and cap 256
• preserves the existing proxy hardening (keepAliveTimeout: 1, keepAliveMaxTimeout: 1, pipelining: 0)
• documents the env var in .env.example and docs/reference/ENVIRONMENT.md
• adds unit coverage for default, diagnostic single-connection override, and cap behavior

Validation

• node --import tsx --import ./open-sse/utils/setupPolyfill.ts --import ./tests/_setup/isolateDataDir.ts --test tests/unit/proxy-dispatcher-family.test.ts
• npm run test:fixes
• node --import tsx --import ./open-sse/utils/setupPolyfill.ts --import ./tests/_setup/isolateDataDir.ts --test tests/unit/proxy-fetch.test.ts tests/unit/proxy-egress-isolation-bdd.test.ts tests/unit/proxyfetch-vercel-relay-2743.test.ts tests/unit/provider-limits-apikey-proxy-context.test.ts
• npm run typecheck:core
• npm run check:env-doc-sync

[gemini-code-assist]

Code Review

This pull request introduces the OMNIROUTE_PROXY_DISPATCHER_CONNECTIONS environment variable to configure the maximum concurrent sockets per cached HTTP/SOCKS proxy dispatcher, defaulting to 32 and capped at 256. It updates open-sse/utils/proxyDispatcher.ts to parse, validate, and apply this limit, and adds corresponding unit tests in tests/unit/proxy-dispatcher-family.test.ts along with documentation in .env.example and docs/reference/ENVIRONMENT.md. There are no review comments, and I have no additional feedback to provide as the implementation is clean and well-tested.

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[diegosouzapw]

Thanks @wilsonicdev! 🙏 Merged into release/v3.8.30. Configurable per-proxy connection pooling (OMNIROUTE_PROXY_DISPATCHER_CONNECTIONS, default 32, capped at 256) is exactly what long-lived SSE streams like Codex /v1/responses need to avoid bottlenecking through a single cached proxy dispatcher — and it composes well with the dispatcher diagnostics we just shipped (#4281). Validated locally (11/11 dispatcher tests + typecheck + file-size gate green). Will ship in the next release.