Addressed performance issue on Bulk Copy API with batch insert

[muskan124947]

Description

A GitHub issue was raised highlighting a performance degradation in the Bulk Copy API when using batch inserts
Pre-release 13.1 is slower #2732

The original issue reported was Bulk copy for batch insert destroys data without error #2669
This occurred when inserting string data using the following settings:

- useBulkCopyForBatchInsert=true
- sendStringParametersAsUnicode=false

Under these conditions, data was inserted in byte array format rather than as readable strings.

Expected behavior
Select StateCode from states -> "OH"

Actual behavior
Select StateCode from states; -> "[B@4b01d9e4"

On top of this a new issue was reported Accented characters saved incorrectly in database when using useBulkCopyForBatchInsert=true and sendStringParametersAsUnicode=false #2721.
When inserting data using useBulkCopyForBatchInsert=true and sendStringParametersAsUnicode=false, names with accented characters were saved incorrectly in the database.

Root Cause

When inserting string data using batch insert for bulk copy with below combinations:

- useBulkCopyForBatchInsert=true
- sendStringParametersAsUnicode=false

Fix#2704 and Fix#2727 introduced a performance slowdown because of unnecessary conversion from byte array back to string during the bulk copy batch insert operation.

Fix

Modified the logic to avoid sending data as a byte array in the first place when useBulkCopyForBatchInsert=true. As data inserted to destination table is being taken from dtv.value which is byte array.
Post this fix performance has improved significantly.

Testing

• Added test cases for all combinations of useBulkCopyForBatchInsert and sendStringParametersAsUnicode to validate data insertion (including Unicode and accented characters) without performance degradation.
• Added a test case to simulate an unsupported INSERT-SELECT batch insert with useBulkCopyForBatchInsert=true and sendStringParametersAsUnicode=false, verifying that the fallback to normal execution occurs and the data is inserted correctly.

[codecov]

Codecov Report

❌ Patch coverage is 0% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 51.70%. Comparing base (44d6010) to head (3fc45fc).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
...rc/main/java/com/microsoft/sqlserver/jdbc/dtv.java	0.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #2735      +/-   ##
============================================
+ Coverage     51.60%   51.70%   +0.09%     
- Complexity     4083     4106      +23     
============================================
  Files           149      149              
  Lines         34242    34239       -3     
  Branches       5719     5718       -1     
============================================
+ Hits          17672    17702      +30     
+ Misses        14104    14092      -12     
+ Partials       2466     2445      -21     

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