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

[lukekim]

What

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

resp, err := spice.Search(ctx, &gospice.SearchRequest{
    Text:     "tickets to Tokyo",
    Datasets: []string{"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.

Why

/v1/search works on a default spice run and is a distinctive Spice capability, but Go 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 decode to nil maps — safe to read from in Go

Part of aligning search support across the SDKs.

Verification

• go build ./...
• go vet ./...
• gofmt clean
• Unit tests pass — go test -run TestSearch ./..., covering request encoding, method/path, response decoding (including omitted objects), validation, and runtime error surfacing. All run against httptest, so they need no live runtime.
• 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

[Copilot]

Pull request overview

Adds first-class Go SDK support for the runtime’s POST /v1/search endpoint by introducing SpiceClient.Search plus request/response types, tests, and README documentation so Go users can perform vector, keyword, and hybrid search without hand-rolling HTTP.

Changes:

• Introduces SearchRequest, SearchResponse, and SearchMatch wire-format types and SpiceClient.Search implementation.
• Adds unit tests covering request encoding, method/path, response decoding (including omitted objects), validation, and runtime error surfacing.
• Documents Search usage and behavior in the README, including hybrid search via Keywords.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
search.go	Adds SpiceClient.Search and associated request/response types plus runtime error-message handling.
search_test.go	Adds httptest-based unit tests for encoding/decoding, validation, and error surfacing.
README.md	Documents the new Search API with examples and field behavior notes.

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