fix(combo): isolate session stickiness by combo

[hydraxman]

Summary

• keep session-stickiness bindings local to the Combo that learned them
• preserve same-Combo prompt-cache affinity while preventing identical first messages from reordering another priority chain
• apply the same namespace on standard and round-robin dispatch paths

Related Issues

• Fixes [BUG] Priority combo skips intermediate target and falls through to later target #10136

Validation

• Change type: routing
• Focused tests and category gates from the golden path
• Targeted ESLint on all changed TypeScript files
• Reconciled with the current active release base; focused checks rerun afterward
• Production-code changes include an updated automated test in this PR
• SonarQube PR analysis is green or any remaining issues are explicitly documented below

Focused validation:

• node --import tsx/esm --test tests/unit/combo-session-stickiness.test.ts tests/unit/combo-disable-session-stickiness.test.ts — 34 passed
• npx eslint open-sse/services/combo.ts open-sse/services/combo/sessionStickiness.ts open-sse/services/combo/targetResolution.ts tests/unit/combo-session-stickiness.test.ts --no-cache --suppressions-location config/quality/eslint-suppressions.json — passed
• npm run check:complexity — passed
• git diff HEAD^ HEAD --check — passed

Tests Added Or Updated

• tests/unit/combo-session-stickiness.test.ts
  • proves bindings remain reusable within one Combo
  • proves the same first message cannot carry that binding into a different Combo

Coverage Notes

The focused regression exercises the new namespace at the session-stickiness boundary and verifies both isolation and retained same-Combo reuse. The two production callers pass the existing Combo name into that boundary; no routing branch or failure-handling path was removed.

Reviewer Notes

The active release head currently has unrelated baseline failures: core typecheck stops at malformed syntax in src/shared/constants/providers/apikey/gateways.ts; file-size reports six untouched files; changelog integrity reports two untouched feature fragments. The changed files pass targeted ESLint, focused tests, complexity, and diff checks.

[diegosouzapw]

Merged into release/v3.8.50 — thank you @hydraxman for the contribution! It was validated on a combined merge-train (static gates + affected tests + vitest) together with 30 sibling PRs before landing.