Fix time-series requests with explicit response units

[krowvin]

Summary

• replace explicit-unit validation that scanned the unbounded AV_TSV_DQU data view with native-to-response unit convertibility validation
• preserve the existing invalid-unit response contract
• add direct-read parity coverage for converted response units

The prior validation issued up to two existence queries against all historical rows for a time series without applying the requested begin/end range. For long-lived series, Oracle could spend the full CDA request timeout validating the unit before executing the bounded data query.

Related Issue

Closes #1901

Validation

• :cwms-data-api:compileJava, compileTestJava, checkstyleMain, and checkstyleTest
• TimeSeriesDirectReadParityIT.convertedUnitReadMatchesRetrieveTs against latest-dev Oracle
• TimeseriesControllerTestIT.test_wrong_units against latest-dev Oracle
• broad gradlew build: 734 tests passed, 37 skipped; 9 unrelated Mockito tests failed because the local Java 21 runtime is newer than the repository's pinned Byte Buddy support (Java 19)

The exact reported two-day payload did not time out against the latest-dev Oracle 23.5 test database. Database schema comparison found the relevant view definition unchanged across recent CWMS database releases; optimizer version, statistics, and historical row volume can explain why the deployed MVP database exhibits the timeout.

Checklist

• AI tools used

[krowvin]

Job failure are due to power outage. HEC Nexus is down.

[MikeNeilson]

Seems like a reasonable change.

[krowvin]

I had drafted while I waited on the HEC box to come back. If you approve I'll go ahead and merge it in.

Unless you're wanting to give it a minute for others to see?