Add system test for picosecond precision timestamps

[danieljbruce]

This change adds a new system test case to system-test/managed_writer_client_test.ts. The test verifies that JSONWriter can successfully append rows containing timestamps with picosecond precision when the table schema is configured with timestampPrecision: 12.

Key changes:

• Defined a new test case "should invoke appendRows with picosecond precision timestamp without errors".
• Created a custom schema with timestampPrecision: 12.
• Implemented table creation, stream setup, and row appending logic mirroring the standard appendRows test.
• Verified that the code compiles and unit tests pass. (Note: system tests were attempted but require live GCP credentials).

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PR created automatically by Jules for task 2947367125746725795 started by @danieljbruce

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