fix(sse): honor CLIProxyAPI environment API key

[RaviTharuma]

Summary

• Honor CLIPROXYAPI_API_KEY when no persisted cliproxyapi_api_key is configured.
• Keep the persisted setting as the higher-precedence source.
• Preserve the environment fallback when settings reads fail.
• Document the server-side environment variable.

Related Issues

• Related to fix(providers): Fallback-to-CLIProxyAPI sends the failed provider's credential instead of a dedicated CLIProxyAPI key → always 401 #7645

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/unit/cliproxyapi-dedicated-credential-7645.test.ts: 7/7 passed
• npm run test:vitest: 463/463 passed
• npm run typecheck:core: passed
• npm run check:cycles: passed
• npm run check:public-creds: passed
• npm run check:docs-all: base currently fails because newly introduced OMNIROUTE_SKIP_NATIVE_DEP_CHECK is missing from the environment contract; this PR adds no references to that variable
• Full unit run reached unrelated existing performance/runner failures and hung in warmup-scheduler tests; the changed focused suite is green

Tests Added Or Updated

• tests/unit/cliproxyapi-dedicated-credential-7645.test.ts
  • environment fallback is trimmed and returned when settings are unavailable
  • passthrough execution uses the environment key when the persisted key is blank
  • the existing no-dedicated-key compatibility path explicitly runs without the ambient environment key

Coverage Notes

• The updated credential suite exercises the resolver and the executor boundary used by direct passthrough, per-connection passthrough, and fallback routing.
• Coverage did not move down in the touched production files.

Reviewer Notes

• The environment variable is a server-side fallback only. It is never logged or returned.
• Existing behavior remains unchanged when neither the setting nor environment variable is configured.