Add timestamp-nanos avro logical type support in bigquery avro utils.

[claudevdm]

timestamp-nanos avro logical type was introduced in avro 1.12, but beam cannot upgrade avro versions until java 8 support is dropped #34993

There are workarounds here to handle the timestamp-nanos logical type until we can update avro version.

AvroUtils changes:

• toBeamFieldType(): Recognizes timestamp-nanos property and maps to Timestamp.NANOS
• toAvroSchema(): Converts Timestamp.NANOS to Avro schema with timestamp-nanos logical type
• toGenericRecord(): Converts Instant values to nanoseconds since epoch
• toBeamRowStrict(): Converts nanosecond values back to Instant objects

BigQueryAvroUtils changes:

• Refactored timestamp formatting into TimestampPrecision enum
• Added NANOSECONDS precision level alongside existing MILLISECONDS and MICROSECONDS
• Updated convertRequiredField() to handle timestamp-nanos logical type
• Updated formatDatetime() to dynamically format fractional seconds (3, 6, or 9 digits)
• Updated fromGenericAvroSchema() to map timestamp-nanos to BigQuery TIMESTAMP type

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[claudevdm]

/gemini review

[gemini-code-assist]

Code Review

This pull request adds support for the timestamp-nanos Avro logical type as a workaround until the Avro version can be upgraded. The changes are primarily in AvroUtils and BigQueryAvroUtils. The implementation is functionally correct and includes good test coverage for edge cases. However, some parts of the implementation rely on implicit behaviors of java.time.Instant, which can be obscure. I've provided a few suggestions to make the code more explicit and maintainable. Additionally, I've suggested a cleaner way to construct an Avro schema programmatically instead of parsing a JSON string.

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[claudevdm]

R: @Abacn
R: @ahmedabu98

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