Add market stream reconnect backoff tests

[qingfeng312]

Summary

Adds a small testable reconnect backoff helper for v2/services/market_stream.rb and focused Minitest coverage for the required delay behavior without loading network dependencies.

/claim #3
Closes #3

Changes

• Added MarketStreamReconnectBackoff.delay with production-equivalent defaults when called from MarketStreamClient#schedule_reconnect.
• Added tests for initial delay, exponential growth, maximum cap, jitter lower/upper bounds, and max-cap behavior with jitter.
• Kept tests isolated from EventMachine, Sinatra, Redis, and real network connections by requiring only the helper module.
• Included the generated diagnostic JSON from python3 build.py; no encrypted .logd artifact was produced locally.

Testing

• ruby v2/services/market_stream_reconnect_backoff_test.rb
• ruby -c v2/services/market_stream.rb
• ruby -c v2/services/market_stream_reconnect_backoff.rb
• git diff --check
• python3 build.py generated diagnostic/build-2b54872c.json; frontend and v2-market-stream passed, while backend/market/engine/compliance/nfc/openapi modules failed locally because required toolchains or platform-specific build support were unavailable (cargo, go, cmake, Java runtime, luac, ghc, and macOS MAP_HUGETLB).

Checklist

• Relevant modules affected by these changes build locally
• Tests pass locally
• Diagnostic build JSON is committed in this PR
• Diagnostic .logd artifact is committed in this PR. It was not created by the local build run; the included diagnostic JSON records the failure.
• Changes are scoped to the PR purpose and avoid unrelated cleanup
• Security, privacy, and error-handling implications have been considered