ref: Add backoff to summary fetch retries

[Aaron (btaaron)]

Summary

Adds exponential backoff between existing JS SDK get_json retries to help post run experiment.summarize() calls to /experiment-comparison2, where immediate retries can repeat during the same transient 504/timeout window. Keeps existing retry count and only spaces out retries for callers already opting into get_json retries

Test Plan

• pnpm run build
• pnpm exec vitest run src/logger.test.ts
• pnpm exec vitest run tests/auto-instrumentations/loader-hook.test.ts
• pnpm run test
• pnpm run check:typings

CI note

integration-tests/e2e and e2e-canary are failing on the Copilot/canary e2e suite, which is also failing on main; JS checks for this PR are green