VectorSessionFunctions Cleanup

[kevin-montrose]

Removes two hacks from VectorSessionFunctions

• All the alignment stuff, Tsavorite guarantees value alignment now
• Mutating VectorInput, VectorOutput now receives updated values and VectorInput is readonly

[Copilot]

Pull request overview

This PR refactors the vector-store session function plumbing to remove legacy alignment workarounds and to stop mutating VectorInput/VectorOutput as a side-channel for communicating updated sizes, replacing that with a new “updated desired size” value on VectorOutput.

Changes:

• Removed the alignment/pinning “hack” code path from VectorSessionFunctions and adjusted record sizing calculations accordingly.
• Made VectorInput a readonly struct with init-only properties, and added VectorOutput.UpdatedReadDesiredSize to propagate unknown-size reads across pending completion paths.
• Updated vector migration, DiskANN callbacks, and vector set tests to use the new initialization/propagation patterns.

Reviewed changes

Copilot reviewed 9 out of 9 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
test/standalone/Garnet.test.vectorset/RespVectorSetTests.cs	Updates tests to build VectorInput via object initializers (compatible with init-only fields).
libs/server/VectorOutput.cs	Adds UpdatedReadDesiredSize to carry back size requirements for unknown-size reads.
libs/server/Storage/Functions/VectorStore/VectorSessionFunctions.cs	Removes alignment-related logic and switches unknown-size read propagation from mutating VectorInput to setting VectorOutput.UpdatedReadDesiredSize.
libs/server/Resp/Vector/VectorManager.Migration.cs	Removes AlignmentExpected usage for migration reads.
libs/server/Resp/Vector/VectorManager.cs	Removes the CompletePending overload that previously propagated mutated VectorInput.
libs/server/Resp/Vector/VectorManager.ContextMetadata.cs	Updates metadata write path to construct VectorInput with init-only properties.
libs/server/Resp/Vector/VectorManager.Callbacks.cs	Updates DiskANN callback paths to use the new VectorInput initialization and UpdatedReadDesiredSize retry mechanism.
libs/server/InputHeader.cs	Makes VectorInput readonly with init-only properties and removes AlignmentExpected.
libs/cluster/Server/Migration/MigrateOperation.cs	Removes AlignmentExpected when migrating vector set element data.

[Copilot]

Pull request overview

Copilot reviewed 9 out of 9 changed files in this pull request and generated 4 comments.

Comments suppressed due to low confidence (1)

libs/server/InputHeader.cs:632

• VectorInput is now a readonly struct with auto-properties, but the explicit parameterless ctor body is empty. In C#, an explicit struct ctor must definitely assign all instance fields; with auto-properties this typically fails compilation unless you delegate to the default initializer. Either remove this ctor or delegate to : this() / this = default so all backing fields are initialized.

        public VectorInput()
        {
        }

[Copilot]

Pull request overview

Copilot reviewed 9 out of 9 changed files in this pull request and generated 2 comments.

[TedHartMS]

I'm not clear whether we're ensuring Key size is aligned to (now removed) ValueAlignmentBytes or if when we support ExtendedNamespace we'll be guaranteeing ExtendedNS+Key lengths are aligned to ValueAlignmentBytes; if not then we may still need to align the value start

[kevin-montrose]

That's a good point - today we'd get away with alignment because DiskANN provided keys are (generally) all 4-bytes. But this would make that contractual.

I'll discuss and come back with a plan.

[kevin-montrose]

We'll update DiskANN to guarantee keys are always of alignment friendly sizes.

I'll set this back on draft until we've got a version to bump to that does that.