Complete Vector feature v2 (float16) serialization and bulk copy support

[Shiwani Gupta (shiwanigupta0809)]

Description

Completes Phase 2 of Vector feature v2 (float16) support in mssql-tds. Phase 1
(feature negotiation and deserialization) landed in PR 7493 and left
todo!("Phase 2: Float16 serialization") in the value serializer, so float16
vectors could be read from the server but not written back. This PR implements
the write path and enables float16 by default.

Serialization — tds_value_serializer.rs replaces the todo!() with a real
implementation that narrows f32 values to IEEE 754 half-precision using
round-to-nearest-even (via the half crate), matching the msodbcsql reference
driver's Float32ToFloat16Bits. Elements are written little-endian, 2 bytes each,
behind the standard 8-byte vector header.

Bulk copy — bulk_copy_metadata.rs now emits vector(N, float16) in generated
DDL when the column's base type is float16. Previously it always emitted
vector(N), which SQL Server interprets as float32, so bulk copy of float16
columns produced a type mismatch.

Default version — client_context.rs flips the default VectorVersion from
V1 to V2, resolving the // TODO: make V2 as default when full V2 support is added left by Phase 1. Version negotiation is unchanged: the client advertises
v2, and the server may still ack v1, in which case behavior is identical to today.

Tests

• 3 serializer unit tests covering float32 elements, float16 elements, and
  round-to-nearest-even behavior (0.1f32 → 0x2E66)
• 2 bulk copy metadata tests for float16 dimension math and DDL generation
• 2 SqlVector tests for try_from_f16 and the higher float16 dimension ceiling
  (3996 vs. 1998)
• 1 feature negotiation test for a v2 ack
• 14 float16 integration tests covering query, parameter binding, output
  parameters, NULL handling, max dimensions, precision loss, mixed base types,
  and bulk copy

mssql-tds/tests/common/mod.rs now honors a DB_DATABASE env var so integration
tests can target a database with PREVIEW_FEATURES = 1 enabled, defaulting to
master as before.

Related Issues

https://sqlclientdrivers.visualstudio.com/mssql-rs/_workitems/edit/45020
https://sqlclientdrivers.visualstudio.com/mssql-rs/_workitems/edit/45021

Checklist

• cargo bfmt passes
• cargo bclippy passes
• cargo btest passes
• New/changed functionality has tests
• Public API changes are documented

[Copilot]

Pull request overview

Completes float16 Vector v2 write support in mssql-tds.

Changes:

• Adds float16 serialization and bulk-copy metadata.
• Defaults vector negotiation to v2.
• Expands unit and integration coverage.

Reviewed changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
mssql-tds/src/connection/client_context.rs	Defaults vector negotiation to v2.
mssql-tds/src/datatypes/bulk_copy_metadata.rs	Generates float16 vector DDL.
mssql-tds/src/datatypes/sql_vector.rs	Tests float16 construction and limits.
mssql-tds/src/datatypes/tds_value_serializer.rs	Serializes float16 elements.
mssql-tds/src/message/features/vectorfeature.rs	Tests v2 acknowledgement.
mssql-tds/tests/common/mod.rs	Supports configurable test databases.
mssql-tds/tests/test_bulk_copy_vector.rs	Adds float16 bulk-copy tests.
mssql-tds/tests/test_vector_type.rs	Adds float16 integration tests.

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