[$20 BOUNTY] feat: add tests for market stream reconnect backoff

[shadow6427]

Summary

Resolves the market stream backoff test bounty ($20).

Changes

• Extracted BackoffHelper.calculate_delay from MarketStreamClient#schedule_reconnect without changing public stream behavior.
• Retained the original production default behaviors (no jitter, same maximum cap).
• Added tests/market_stream_test.rb covering initial delay, exponential growth, maximum cap, and jitter bounds.
• Ensure the tests do not require external network access.

Validation

To validate locally without starting the whole suite, you can run:
ruby -I. v2/services/market_stream_test.rb
(Ensure minitest is available or run within bundle).

Checklist

• Backoff helper extracted
• Initial delay, exponential growth, maximum cap, and jitter bounds covered
• Production defaults unchanged
• Tests do not require external network access
• Documented validation command