Fix rate provider correctness edge cases

[entity]

Summary

• Validate and normalize requested base currencies before cache keys and provider requests are built.
• Reject provider responses whose base/source metadata does not match the requested base, preventing silent wrong conversions and cache pollution.
• Harden edge cases around empty rate payloads, custom-base cache flushing, ECB reference updates, invalid conversion amounts, and fallback driver validation.

Tests

• composer lint
• composer test