SqliteVec: re-use a single command for upserts

[rossdonald]

Changes upserts to use a single command for inserts so SQL text for an insert is stable across records and the provider's per-command prepared statement cache and parameter bindings can be reused, and only values are re-bound per record.
Fixes #54

[Copilot]

Copilot review overview

🟡 Changes recommended

Remove the unused test declarations causing CS0219 and bump the provider package version.

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Review effort: Lite
Findings: 1

Open (1)

• Remove unused records local causing CS0219 · New

What changed in this PR

Refactors SqliteVec upserts to reuse stable single-row commands and rebind values per record.

Changes:

• Reuses prepared insert commands.
• Separates returning and non-returning upsert paths.
• Expands command-builder tests for parameter rebinding and vector handling.

File	Summary
MEVD/​test/​SqliteVec.UnitTests/​SqliteCommandBuilderTests.cs	Tests updated command and parameter-binding behavior; unused declarations cause CS0219.
MEVD/​src/​SqliteVec/​SqliteCommandBuilder.cs	Builds stable insert SQL and binds values dynamically; package version should be bumped.
MEVD/​src/​SqliteVec/​SqliteCollection.cs	Reuses insert commands during upserts; package version should be bumped.

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[Copilot]

Copilot review overview

🔵 Needs a closer look

Bump the provider package version for the implementation changes.

Review effort: Lite
Findings: None

Resolved since last review (1)

• Remove unused records local causing CS0219

Previously missed (1)

In code that hasn't changed since last review

Update provider version for changed C# code

MEVD/​src/​SqliteVec/​SqliteCommandBuilder.cs:120

This PR changes provider implementation files under MEVD/src/SqliteVec, but MEVD/src/SqliteVec/SqliteVec.csproj still declares version 1.0.2-preview. The repository release guidance requires a SemVer update for every such C# change; please bump the provider version so this fix can be published under a distinct package/tag version.

[adamsitnik]

@rossdonald thanks for another great contribution!

The code looks good, I found only few minor things that could be addressed before we merge, PTAL at my comments.

[rossdonald]

Thanks for the feedback, looks good. I will update in a few days as I am travelling

[Copilot]

Copilot review overview

🟡 Changes recommended

Two moderate issues and one versioning nit remain unresolved.

Review effort: Lite
Findings: 1

Open (1)

• Bump provider package version after implementation changes · New

[adamsitnik]

It looks great, thank you for another awesome contribution @rossdonald !

Batch size	Base mean	PR mean	Time change	Base allocation	PR allocation	Allocation change
1	1.149 ms	1.165 ms	1.4% slower (noise-level)	12.84 KB	12.47 KB	2.9% lower
10	1.354 ms	1.260 ms	6.9% faster	72.51 KB	29.98 KB	58.7% lower
100	5.927 ms	2.054 ms	65.3% faster	3178.35 KB	205.05 KB	93.5% lower