fix(bigquery): [B-5] fix self-assignment and wrong-getter bugs in domain models

[lqiu96]

Summary & Bottleneck

Several domain model classes in google-cloud-bigquery contain self-assignment, wrong-getter, and switch fall-through bugs discovered during the codebase audit:

1. IndexUnusedReason.toPb() self-assignment:
   • In java-bigquery/google-cloud-bigquery/src/main/java/com/google/cloud/bigquery/IndexUnusedReason.java, toPb() calls indexUnusedReason.setIndexName(indexUnusedReason.getIndexName()), indexUnusedReason.setCode(indexUnusedReason.getCode()), and indexUnusedReason.setMessage(indexUnusedReason.getMessage()) on the newly allocated empty protobuf instance instead of reading getIndexName(), getCode(), and getMessage() from this.
2. JobStatistics.Builder(JobStatistics) uninitialized field check:
   • In java-bigquery/google-cloud-bigquery/src/main/java/com/google/cloud/bigquery/JobStatistics.java, Builder(com.google.api.services.bigquery.model.JobStatistics statisticsPb) checks if (reservationUsage != null) (which is always null on a newly constructed Builder) instead of if (statisticsPb.getReservationUsage() != null), causing getReservationUsage() to always return null after fromPb.
3. ExternalTableDefinition.fromExternalDataConfiguration wrong getter:
   • In java-bigquery/google-cloud-bigquery/src/main/java/com/google/cloud/bigquery/ExternalTableDefinition.java, fromExternalDataConfiguration calls builder.setTimestampFormat(externalDataConfiguration.getTimeFormat()) instead of externalDataConfiguration.getTimestampFormat().
4. QueryParameterValue duplicate String.class branches and case JSON: fall-through:
   • In java-bigquery/google-cloud-bigquery/src/main/java/com/google/cloud/bigquery/QueryParameterValue.java, classToType contains two unreachable duplicate else if (String.class.isAssignableFrom(type)) branches, and valueToStringOrNull is missing a break statement at the end of case JSON:, allowing non-JSON PeriodDuration values to fall through into case INTERVAL:.

Changes

• java-bigquery/google-cloud-bigquery/src/main/java/com/google/cloud/bigquery/IndexUnusedReason.java:
  • Pass getIndexName(), getCode(), and getMessage() to the protobuf setters in toPb().
• java-bigquery/google-cloud-bigquery/src/main/java/com/google/cloud/bigquery/JobStatistics.java:
  • Check if (statisticsPb.getReservationUsage() != null) in JobStatistics.Builder(statisticsPb).
• java-bigquery/google-cloud-bigquery/src/main/java/com/google/cloud/bigquery/ExternalTableDefinition.java:
  • Pass externalDataConfiguration.getTimestampFormat() to builder.setTimestampFormat(...) in fromExternalDataConfiguration.
• java-bigquery/google-cloud-bigquery/src/main/java/com/google/cloud/bigquery/QueryParameterValue.java:
  • Remove the unreachable duplicate String.class branches in classToType and add the missing break statement in case JSON: of valueToStringOrNull.

Expected Impact

• Restores lossless toPb() and fromPb() round-trip serialization for IndexUnusedReason, JobStatistics.getReservationUsage(), and ExternalTableDefinition.getTimestampFormat().
• Prevents invalid PeriodDuration values from being silently accepted when constructing StandardSQLTypeName.JSON query parameters.

Verification

• Added unit tests in java-bigquery/google-cloud-bigquery/src/test/java/com/google/cloud/bigquery/JobStatisticsTest.java, ExternalTableDefinitionTest.java, and QueryParameterValueTest.java.
• Ran mvn test -pl java-bigquery/google-cloud-bigquery -Dtest=JobStatisticsTest,ExternalTableDefinitionTest,QueryParameterValueTest (51 tests run, 0 failures).