fix(proxies): shorten refused-egress set-aside and rest slow egress

[maxmad64bis]

⚠️ base-red inherited: #14963

Summary

Default change: PROXY_QUOTA_429_BASE_MS 120s → 300s, PROXY_QUOTA_429_MAX_MS 3600s → 900s — a refused egress rarely recovers within minutes, while the lower cap lets a recovered member return within the quarter-hour.
Repeatedly refused egresses were parked for up to an hour, and egresses stalling on response headers were never rested at all. This PR shortens the refused-egress curve to 300s → 600s → 900s and adds a short 60s/600s slow curve that sets an egress aside after three settled headers-waits in five minutes under the same opt-in flag; the pool screen labels that motive ("slow responses") instead of printing the raw kind. Three existing default tests were adapted to the new values.

Related Issues

• No linked issue — no public issue tracks this tuning.

Validation

• Change type: routing
• Focused tests and category gates from the golden path
• npm run lint
• Reconciled with the current active release base; focused checks rerun afterward
• Production-code changes include a new or updated automated test in this PR

Tests Added Or Updated

• tests/unit/proxy-slow-overrun.test.ts (new): k=3/W=5min gating, lone/pair without effect, null key, flag off, expired window, served-clears-slow, 60s/600s curve, 20min purge, store size.
• tests/unit/proxy-refusal-memory.test.ts (adapted): curve 5/10/15/15min, purge 29/30min.
• tests/unit/proxy-refusal-memory-quota-tuning.test.ts (adapted): default periods and fallback expectations.
• tests/unit/opencode-proxy-refusal-memory.test.ts (adapted): a refused member is still set aside after the former 2-minute default and returns after 5 minutes.
• tests/unit/ui/PoolMemberEgressLines.test.tsx (1 new case): a slow set-aside is labelled, never printed raw.

Coverage Notes

The new store counter, gated helper, and expiry call site in open-sse/ are exercised by the new slow-overrun tests, and the adapted tests pin the new default curve; the two widened reason unions are type-only with no runtime path.

Reviewer Notes

• Default declared above (120s→300s base, 3600s→900s cap). This shortens how long a refused egress stays parked; a request can still fail without any upstream call while every egress is set aside, and this change only shortens that window.
• Shares open-sse/utils/proxyRefusalMemory.ts with fix(proxies): steer selector to members without recent refusals #14804 and open-sse/executors/opencode.ts with fix(opencode): bound the opt-in Responses first-byte stall window by the stream readiness timeout #14940 (disjoint hunks) — whichever lands first, the others rebase. The one-line comment reflow in opencode.ts keeps that frozen file at its size cap.
• No rebaseline and no new complexity violation. Inherited from the base (🔴 Release branch not green: release/v3.8.51 #14963): mutation-test-coverage --strict fails the same way on the release base; no file of this PR is involved. The new label key is translated in all 66 locales.

Maintainer rework (merge-batch 2026-09-28)

• Confirmed that the rebase pushed after the first review changes no content. The patch matches the one already validated.
• Updated .env.example so the PROXY_QUOTA_429_MAX_MS example shows the new 900000 cap instead of the old 3600000.
• Merged release/v3.8.51 into the branch (real merge, no conflicts).
• Validation on the merged head: the 4 refusal-memory and slow-overrun suites pass 43/43, and PoolMemberEgressLines passes 7/7 under vitest. typecheck:core, check:open-sse-typecheck, eslint (with the suppressions file) and check:env-doc-sync are all clean. The two check-file-size reds (SystemStorageTab.tsx and response-sanitizer.test.ts) are inherited from the tip; this PR does not touch either file.