babelfish-for-postgresql / babelfish_extensions / 19462655229

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Implement bidirectional cast/convert for char/nchar ↔ binary/varbinary

This commit introduces support for casting/converting between char/nchar types
and binary/varbinary, improving compatibility with T-SQL-style conversions in
Babelfish.

What is the change?
Previously, Babelfish did not support direct casts or converts between char/
nchar and binary/varbinary types. With this change, the system can now handle
such casts bidirectionally, enabling more seamless T-SQL compatibility.

Why was the change made?
This change was driven by the need to increase compatibility with T-SQL’s
casting behavior, as part of issue #3835. T-SQL workloads often rely on
implicit and explicit casts between string and binary types, so supporting
these casts is essential for broader T-SQL compatibility.

How was the code changed?

babelfish_conv_helper_to_nvarchar covers conversions to nvarchar.
Implemented and registered cast functions from nchar to binary/varbinary.
Implemented and registered cast functions from binary/varbinary to char/nchar.
Adjust bpchar/varchar to binary cast functions and use encoding.
Updated conversion helper logic.
Added test cases covering each cast direction.
Add missing support for TRY_CAST, CONVERT and TRY_CONVERT for (nvar)char & 
var(binary).

Issues Resolved:
BABEL-6084
https://github.com/babelfish-for-postgresql/babelfish_extensions/issues/3835

Author: Bilal Kahraman <kahramannbilal@gmail.com>
Reviewed-by: Tanzeel Khan <tzlkhan@amazon.com>
Reviewed-by: Rohit Bhagat <rohitbgt@amazon.com>
Reviewed-by: Pranav Jain <pranav23iitd@gmail.com>

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