fix: embedding deserialization breaks with serde_json/arbitrary_precision

[EinarasGar]

Problem

Embedding deserialization fails when serde_json's arbitrary_precision feature is enabled anywhere in the dependency tree. The error is:

JsonError: data did not match any variant of untagged enum ApiResponse

This is commonly triggered by rust_decimal with the serde-with-arbitrary-precision feature, which is widely used in financial applications.

Root cause

When serde_json/arbitrary_precision is active, all JSON numbers are internally represented as strings. The Vec<f64> fields in embedding response structs (EmbeddingValues, EmbeddingData, etc.) fail to deserialize because serde expects native floats but receives string-backed numbers.

The arbitrary_precision feature is a compile-time, crate-wide flag — it cannot be opted out of per-deserialization call. Any crate in the dependency tree that enables it (directly or transitively) affects all serde_json usage in the binary.

Reproduction

Any project that combines rig embeddings with rust_decimal:

[dependencies]
rig-core = "0.32"
rust_decimal = { version = "1", features = ["serde-with-arbitrary-precision"] }

let client = gemini::Client::new("key").unwrap();
let model = client.embedding_model_with_ndims("gemini-embedding-001", 768);
let result = model.embed_text("hello").await; // fails with JsonError

The same code works in a crate without rust_decimal in the dependency tree.

Fix

Change embedding response structs from Vec<f64> to Vec<serde_json::Number>, then convert to f64 via .as_f64() when building the Embedding struct.

serde_json::Number is the canonical number type that works correctly regardless of whether arbitrary_precision is enabled — it's the internal representation serde_json uses in both modes.

Affected providers

All 6 providers with embedding support:

• Gemini (EmbeddingValues.values)
• OpenAI (EmbeddingData.embedding)
• Mistral (EmbeddingData.embedding)
• OpenRouter (EmbeddingData.embedding)
• Together (EmbeddingData.embedding)
• Cohere (EmbeddingResponse.embeddings)

Change summary

Each provider's embedding response struct: Vec<f64> → Vec<serde_json::Number>, with .filter_map(|n| n.as_f64()) at the point where the Embedding struct is built. No new dependencies, no API changes.