Refactor stream option conversion tests for Lettuce

[Jimin730]

Refactor: Extract Lettuce StreamConverters unit tests

Move stream option-to-args conversion tests into StreamConvertersUnitTests. Simplify LettuceConnectionUnitTests to verify API flow only. Align Lettuce tests with existing Jedis test structure.

No functional changes. Test refactoring only.

Closes #3276

Changes

• New: StreamConvertersUnitTests
• Modified: LettuceConnectionUnitTests - simplify stream-related tests

Testing

• StreamConvertersUnitTests and modified LettuceConnectionUnitTests pass.
• Some existing tests are failing on main and are unrelated to this change.

Checklist

• [x] You have read the Spring Data contribution guidelines.
• [x] You use the code formatters provided here and have them applied to your changes. Don’t submit any formatting related changes.
• [x] You submit test cases (unit or integration tests) that back your changes.
• [x] You added yourself as author in the headers of the classes you touched. Amend the date range in the Apache license header if needed. For new types, add the license header (copy from another file and set the current year only).