feat: add SearchAsync for vector, keyword, and hybrid search

[lukekim]

What

Adds SpiceClient.SearchAsync, wrapping the runtime's POST /v1/search — vector similarity, keyword, and hybrid search over datasets with an embedding column.

var response = await client.SearchAsync(new SearchRequest("tickets to Tokyo")
{
    Datasets = new[] { "app_messages" },
    Limit = 3,
});

Only Text is required. Setting Keywords pre-filters the embedding column with a lexical search before the vector search runs, making the search hybrid.

New public types in Spice.Search: SearchRequest, SearchResponse, SearchMatch.

Why

/v1/search works on a default spice run and is a distinctive Spice capability, but .NET users had no way to reach it short of hand-rolling the HTTP call. spice.js is currently the only SDK that exposes it.

Two wire-format details the types handle so callers don't have to:

• the similarity score is serialized as _score, not score
• data, primary_key and metadata are omitted entirely when empty, so they default to empty dictionaries rather than null — readable without a guard

On a failed search the runtime's own {"error": "..."} message is surfaced rather than a bare status code, since that message names what the caller needs to fix.

Part of aligning search support across the SDKs.

Notes for review

• System.Text.Json is now an explicit PackageReference. It was already resolved transitively through Apache.Arrow.Adbc (which requires >= 9.0.9); this pins the same version explicitly because the search path depends on it directly, and netstandard2.0 has no in-box System.Text.Json. Pinning below 9.0.9 fails the build with NU1605.
• ReadAsStringAsync is #if-split — the CancellationToken overload doesn't exist on netstandard2.0, and the analyzers treat CA2016 as an error on the modern targets.
• This repo has feat: add health and readiness checks #20 (health/readiness) and feat: add refresh options for on-demand dataset refresh #21 (refresh options) open, both touching SpiceClient.cs and the HTTP client. This branch adds new members rather than changing existing ones, but it will want a merge rather than a fast-forward if those land first.

Verification

• dotnet build -c Release — all four targets, netstandard2.0 included
• dotnet test — 125 passed, 0 failed on each of net8.0 / net9.0 / net10.0, including 10 new tests in SearchTest.cs covering request serialization (wire field names, limit: 0, empty collections) and response deserialization (_score, omitted objects, missing keys)
• Verified against a live spice run: the search request reaches /v1/search, and a
  failure surfaces the runtime's own message ("Search cannot be run on <dataset> because it has no embeddings or full text search indexes.") rather than a bare status code
• A search returning matches was not exercised end to end — that needs a dataset with an
  embedding column and a loaded embedding model, which this environment has no credentials for. The 33 skipped tests are the existing suites that require one.

[Copilot]

Pull request overview

Adds first-class .NET SDK support for the Spice runtime /v1/search endpoint via SpiceClient.SearchAsync, including strongly-typed request/response models, HTTP wiring, unit tests for the wire contract, and README documentation so callers can perform vector/keyword/hybrid search without hand-rolling HTTP.

Changes:

• Introduces SpiceClient.SearchAsync and ISpiceHttpClient.SearchAsync to expose /v1/search.
• Adds Spice.Search types (SearchRequest, SearchResponse, SearchMatch) with JSON wire-format handling (e.g., _score, omitted empty objects).
• Adds unit tests for request/response serialization and documents usage in README; pins System.Text.Json explicitly.

Reviewed changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
SpiceTest/SearchTest.cs	Adds tests for request serialization and response deserialization wire contract.
Spice/src/SpiceClient.cs	Exposes new SearchAsync API surface on the public client.
Spice/src/Search/SearchTypes.cs	Defines public search request/response/match DTOs with JSON attributes.
Spice/src/Http/SpiceHttpClient.cs	Implements /v1/search POST, JSON (de)serialization, and error extraction.
Spice/src/Http/ISpiceHttpClient.cs	Adds SearchAsync to the HTTP client interface.
Spice/Spice.csproj	Adds explicit System.Text.Json package reference.
README.md	Documents SearchAsync usage and explains request/response fields.

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