Binary to string mappings

[sm745052]

Description

This PR fixes data corruption issues when mapping source database binary and bit types to Spanner STRING columns, and ensures they successfully round-trip during reverse migrations.

The Problem

Previously, when forward migrating binary/blob/bit data into a Spanner STRING column, the pipeline was calling .toString() on the underlying Java ByteBuffer. This destroyed the actual binary data by inserting literal Java memory references (e.g., "java.nio.HeapByteBuffer[pos=0 lim=5 cap=5]") into Spanner. Consequently, reverse migrations back to the source database were completely broken, and the related integration tests were commented out or ignored.

The Fix

1. Forward Migrations (AvroToValueMapper.java)

• Intercepts ByteBuffer and byte[] types before they are cast to strings.
• Safely encodes the raw binary data into a Hexadecimal String before writing it to Spanner.

2. Reverse Migrations (DML Generators)

• MySQL: Updates MySQLDMLGenerator to detect when a Spanner string is being mapped back to a blob or binary column. It now wraps the generated value in the UNHEX() function (e.g., UNHEX('7835383030')) to natively restore the raw bytes.
• PostgreSQL: Updates PostgreSQLDMLGenerator to map hex-encoded Spanner strings back to bytea by wrapping the output in the decode(..., 'hex') function.
• Bit Handling: Ensures BIT and VARBIT arrays correctly map to strings and migrate back to their native representations without issue.

3. Integration Tests

• Restored and uncommented previously ignored binary-to-string integration tests across MySQLDataTypesIT, PostgreSQLDataTypesIT, and SpannerToMySqlDataTypesIT.
• Updated test expectations to assert successful end-to-end hex encoding round-trips.

[codecov]

Codecov Report

❌ Patch coverage is 83.33333% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 61.90%. Comparing base (9f81d23) to head (2515803).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
...t/v2/templates/source/mysql/MySQLDMLGenerator.java	81.81%	0 Missing and 2 partials ⚠️
...plates/source/postgres/PostgreSQLDMLGenerator.java	66.66%	0 Missing and 2 partials ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #4179      +/-   ##
============================================
+ Coverage     55.90%   61.90%   +6.00%     
+ Complexity     7475     3452    -4023     
============================================
  Files          1134      580     -554     
  Lines         70135    34860   -35275     
  Branches       8014     3878    -4136     
============================================
- Hits          39207    21581   -17626     
+ Misses        28381    12167   -16214     
+ Partials       2547     1112    -1435     

Components	Coverage Δ
spanner-templates	84.63% <83.33%> (-0.02%)	⬇️
spanner-import-export	∅ <ø> (∅)
spanner-live-forward-migration	88.64% <100.00%> (-0.02%)	⬇️
spanner-live-reverse-replication	81.24% <83.33%> (-0.03%)	⬇️
spanner-bulk-migration	89.05% <100.00%> (-0.02%)	⬇️
gcs-spanner-dv	87.85% <100.00%> (-0.03%)	⬇️

Files with missing lines	Coverage Δ
.../v2/spanner/migrations/avro/AvroToValueMapper.java	94.56% <100.00%> (+0.16%)	⬆️
...t/v2/templates/source/mysql/MySQLDMLGenerator.java	95.48% <81.81%> (-1.10%)	⬇️
...plates/source/postgres/PostgreSQLDMLGenerator.java	90.32% <66.66%> (-1.02%)	⬇️

... and 571 files with indirect coverage changes

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