test(bigquery-jdbc): make picosecond ITs timezone independent and disable on TPC

[keshavdandeva]

This PR fixes ITJdbcTimestampPicosTest failures in Kokoro

Changes

• Time zone (integration_test_continuous, standalone_it_continuous)

Four assertions compared hardcoded UTC literals against getString() on plain
TIMESTAMP columns, which the driver renders in the JVM default time zone. The tests
only passed under a UTC JVM; the Kokoro VM runs America/New_York, giving the observed
12:34:56 vs 07:34:56 mismatch.

Added an atJvmZone() helper that converts the UTC literal to the JVM zone, mirroring
the existing pattern in ITBigQueryJDBCTest.validateGetString. TIMESTAMP(12)
assertions are unchanged. Those values are returned as verbatim UTC strings and are
already zone invariant.

• TPC (tpc_integration_tests_continuous)

TPC backends reject the TIMESTAMP(12) type parameter, so @BeforeAll failed and the
whole class errored. Added a class-level @Tag("disable_tpc")

[gemini-code-assist]

Code Review

This pull request updates the ITJdbcTimestampPicosTest class by introducing a helper method, atJvmZone, to convert and format UTC timestamp literals into the JVM's default time zone, which fixes zone-dependent assertion failures. Additionally, the test class is tagged with @tag("disable_tpc"). The review feedback suggests optimizing the helper method by defining the DateTimeFormatter as a private static final constant to prevent redundant object allocations.