feat: add nsql and nsql_generate_sql for the runtime's /v1/nsql endpoint

[claudespice]

What

Adds Client::nsql and Client::nsql_generate_sql, wrapping the runtime's /v1/nsql text-to-SQL endpoint, plus the NsqlRequest / NsqlResponse types in a new nsql module.

Why

Text-to-SQL is reachable from spice.js and from no other SDK. A Rust caller who wanted it had to hand-roll the HTTP request — including knowing to send Accept: application/vnd.spiceai.nsql.v1+json, without which the runtime falls back to a bare array of rows and the generated SQL is lost.

Two methods rather than one, because the generated SQL is useful on its own:

• nsql generates, runs, and returns the rows together with the SQL that produced them.
• nsql_generate_sql stops after generation. That lets a caller inspect or edit the query first, and — since /v1/nsql results arrive as decoded JSON, where Arrow types are flattened to JSON's — run it through sql() instead when Arrow types matter.

NsqlRequest follows the existing SearchRequest shape: new() plus with_* builders taking mut self. Errors are a typed NsqlError enum; a missing or ambiguous model is the most common failure and the runtime explains it in the response body, so that body is surfaced rather than collapsed into a status code.

Part of aligning capability across the SDKs; /v1/search and /v1/status landed here recently and NSQL was the remaining gap against the runtime's client-facing surface.

Verification

• cargo build, cargo fmt --all --check
• cargo clippy --all-features --all-targets — no new warnings (the 5 remaining are pre-existing, in tests/client_test.rs)
• cargo test --lib — 235 pass, including 12 new NSQL cases: request serialization and default-omission, response decoding with both a simple and a parameterized Arrow type, empty result set (the runtime sends schema: {}), and wiremock coverage asserting each media type, the surfaced error body, pre-send validation, and the missing-http_url path
• cargo test --doc — 39 pass, including the two new README examples
• Integration tests — not run; the 7 test_local_* cases in tests/client_test.rs need spice run on the default ports and the spice CLI is not available in this environment.