fix(client): cache authEnvKeys results to reduce allocations

[Yuan-ManX]

No description provided.

[dutifulbob]

Triage result

Human attention: ⚠️ Required
Recommendation: 🏁 escalate to a human
Human decision needed: ready for human landing decision

Quick read

This PR is a small client-side performance optimization that caches derived auth environment-variable keys. Scope is limited to src/client.ts, standard checks were the right validation path, review is clear, CI is green, and both conflict checks are clean.

Intent

Reduce overhead in the client auth path by caching the derived environment-variable key list for each auth method instead of rebuilding it on every lookup.

Why

Credential lookup was repeatedly recomputing the same normalized auth env key arrays for a given method ID, creating unnecessary Set and array allocations during auth selection.

Codex review

No blocking P0/P1 findings remain.
GitHub Codex review findings for this head: none.
Local Codex review was established and reported no actionable correctness issues in the changed auth-key caching path.

CI/CD

CI was initially approval-blocked on workflow run 23401050132.
Workflow approval was applied successfully.
The rerun completed successfully with all relevant jobs green.
Initial and final conflict checks against origin/main were both clean.

Recommendation

This change looks right-shaped and low risk, with no remaining blocking review or CI concerns. It is ready for human landing decision.