fix: prevent proxied stream crashes and normalize Groq reasoning

[kalpakprod]

These fixes were found while running the gateway in production under load.

• Proxy failures: an escaped PROXY_REQUEST_FAILED could terminate the gateway and cut unrelated in-flight streams → classify it as a recoverable upstream network error while preserving fatal handling for genuine programming errors.
• SOCKS streaming: HTTP/2 stream resets could escape through the SOCKS transport → pin SOCKS dispatchers to HTTP/1.1.
• Groq GPT-OSS: foreign reasoning/output_config envelopes and forced none caused invalid requests → strip foreign envelopes and map forced none/low to low for openai/gpt-oss-20b, preserving ordinary model defaults.
• ChatGPT browser sessions: the current ProseMirror composer was absent from the shared selector → recognize its editable textbox alongside legacy editors.

Validation: Node v26.8.1, dependencies installed with the upstream lockfile (npm ci --ignore-scripts --no-audit --no-fund --maxsockets=4). Eight focused upstream unit suites passed: 39 tests, zero failures or skips. Regression assertions fail against the unchanged upstream source and pass with this patch. ESLint and the repository commit hooks passed. Gitleaks and private-data pattern scans passed for the contribution.

node --import tsx/esm --import ./open-sse/utils/setupPolyfill.ts --import ./tests/_setup/isolateDataDir.ts --test --test-force-exit --test-concurrency=1 tests/unit/provider-field-strips.test.ts tests/unit/groq-field-strip-wiring.test.ts tests/unit/http-client-abort-guard-direct-timeout-12861.test.ts tests/unit/socks-connector-family.test.ts tests/unit/chatgpt-composer-selector.test.ts tests/unit/socks-connect-timeout.test.ts tests/unit/socks-connect-timeout-e2e.test.ts tests/unit/socks-handshake-timeout-5109.test.ts

⚠️ base-red inherited: #15306. The focused suites above pass; the complete upstream unit/Vitest matrix was not run locally.

https://claude.ai/code/session_01BsVwQEhCy4c35aK8tjA9A6