fix: stabilize V2 exchange rate channel test

[DenisCarriere]

Summary

• invalidate both MarketHistoryCache entries after inserting test fixtures
• synchronously verify the cache reload returns the expected 30 records before broadcasting
• align the V2 channel test with the stable legacy exchange-rate channel test

Why

The test intermittently broadcasts chart_data as an empty list because a stale or concurrently warmed cache can survive the fixture insert. The same assertion failed in Eden, Blackfort, and the default PR matrix across separate runs.

Validation

• clean Linux compilation passed with mix do deps.get, local.hex --force, local.rebar --force, deps.compile, compile on Elixir 1.19.4 / OTP 27
• mix format --check-formatted passed in the pinned toolchain
• git diff --check passed
• full default Blockscout CI matrix passed
• Blockscout Web suite: 29 doctests, 1708 tests, 0 failures, 1 skipped
• Credo, Dialyzer, Sobelow, CodeQL, JS, Explorer, Indexer, and EthereumJSONRPC checks passed
• 0 unresolved review threads

Refs #9