feat: expose ONNX Runtime session config entries through InitOptions

[paulnsorensen]

Closes #291.

What changed

• init_session_builder takes a session_config: Vec<(String, String)> parameter. It applies each entry with SessionBuilder::with_config_entry after with_intra_threads. The DirectML block is unchanged.
• InitOptions<M> and InitOptionsWithLength<M> gain a session_config field and a with_session_config(key, value) builder method. The default is empty.
• InitOptionsUserDefined, RerankInitOptionsUserDefined, and ImageInitOptionsUserDefined gain the same field and method. The From impls copy the field.
• All ten init_session_builder call sites pass the field through.
• README: one line next to the with_intra_threads example.

Why

Some ONNX Runtime behaviors have a per-session switch only. mlas.disable_kleidiai is one example. On Apple Silicon with SME, ONNX Runtime 1.28.0 keeps packed GEMM buffers in thread-local storage. The buffers never shrink. A long-lived process retains gigabytes after one embedding pass. With with_session_config("mlas.disable_kleidiai", "1") the same process retains 216 MB instead of 2189 MB. See #291 and microsoft/onnxruntime#29538.

The shape follows the existing execution_providers and intra_threads passthroughs.

Usage

let model = TextEmbedding::try_new(
    TextInitOptions::new(EmbeddingModel::SnowflakeArcticEmbedS)
        .with_session_config("mlas.disable_kleidiai", "1"),
)?;

Tests

• init_session_builder_applies_config_entry calls the real SessionBuilder with one entry and asserts success. It loads no model.
• Unit tests assert that with_session_config appends in order and that the From impls carry the field.
• cargo fmt --all -- --check, cargo clippy ... -- -D warnings, and cargo test --lib pass with ort-download-binaries-native-tls,hf-hub-native-tls.

Compatibility

The option structs are #[non_exhaustive], so the new field is not a breaking change for callers that use the builder methods. Callers that construct the structs by field name inside this crate are updated.

🤖 Generated with Claude Code

[Anush008]

Thanks for the contribution @paulnsorensen.

[github-actions]

🎉 This PR is included in version 6.1.0 🎉

The release is available on:

• GitHub release
• v6.1.0

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