docs(routing): LKGP sticks at provider level — align copy with routerStrategy contract

[maxmad64bis]

Summary

The lkgp routing strategy pins to the last known good provider and falls back to the rules strategy when that provider has no healthy candidate in the pool (open-sse/services/autoCombo/routerStrategy.ts). Two doc surfaces over-promised this contract:

• README.md strategy table: "sticky to the last successful target"
• docs/guides/USER_GUIDE.md: "sticks to the last successful model per session" and auto/lkgp: "sticky to the most recently successful target"
• docs/routing/AUTO-COMBO.md: "Last-Known-Good Path (sticky route to last successful target)"

The strategy has never operated at target or model granularity — it filters candidates on c.provider === lastKnownGoodProvider, takes the first non-OPEN one, and delegates to rules otherwise. This PR replaces all three lines with a single accurate phrasing: pins to the last successful provider, then falls back to rules. The widened table cell also required re-padding the surrounding auto/* variant table (formatting only).

Translated docs (docs/i18n/**) are intentionally untouched so the maintainer's translation pipeline can pick the corrected source up.

Related Issues

None — follow-up to my earlier docs-accuracy PRs (#11543, #11545) on the same theme: prose drifting from what ships.

Validation

Change type: routing (docs accuracy for a routing strategy; no behavior change).

The full unit suite, Vitest, the 60% coverage gate, and the production build all run in CI on this PR (#8329):

• Change type: routing (docs-only accuracy fix)
• Focused tests and category gates from the golden path — targeted run of tests/unit/router-strategies.test.ts: 24 pass, 0 fail; check-docs-sync.mjs, check-tracked-artifacts.mjs, check:any-budget:t11 all green
• npm run lint — not runnable in my environment (local node_modules is incomplete); the touched .ts file and changelog fragment are prettier-clean, and CI runs lint on this PR
• Reconciled with the current active release base (release/v3.8.51); focused checks rerun after the last rebase
• Production-code changes include a new or updated automated test in this PR — N/A: no production code changed (tests only added under tests/)

Tests Added Or Updated

• tests/unit/router-strategies.test.ts — 2 tests appended to the existing lkgp section:
  1. falls back to rules when the pool lacks the last known good provider — mono-provider pool whose only provider ≠ lastKnownGoodProvider; asserts the decision matches what getStrategy("rules").select() returns on the same inputs (expected derived from the real strategy, not duplicated).
  2. keeps the first candidate among several targets of the pinned provider — two CLOSED-breaker candidates of the pinned provider with distinct models/connections; asserts the first is kept with candidatesConsidered: 1.
• No production code changed in this PR.

Reproduce: node --import tsx/esm --import ./open-sse/utils/setupPolyfill.ts --import ./tests/_setup/isolateDataDir.ts --test tests/unit/router-strategies.test.ts

Coverage Notes

No changes to src/, open-sse/, electron/, or bin/ — coverage is unaffected.

Reviewer Notes

• The USER_GUIDE.md diff includes 9 re-padded table lines around the edited row, and the AUTO-COMBO.md row is padded to its table's canonical prettier width — pure formatting, computed per-table (these doc files are not prettier-clean as whole files, so a full-file run was deliberately avoided).
• Changelog fragment uses placeholder number 0000 until the PR number exists; will be amended right after opening.