feat(openrouter): Add provider selection and prioritization support

[milutinke]

Add OpenRouter Provider Selection and Prioritization

Description

Implements OpenRouter's Provider Routing API, enabling fine-grained control over which providers handle requests.

Motivation: I personally, and many others (probably) need a more fine-grained control over what model providers we want to use. For an example, my personal use case is only using providers with Zero Data Retention policy and FP8, while sorting for throughput.

Provider Selection Controls:

• order - Provider priority ordering
• only - Hard allowlist
• ignore - Blocklist
• allow_fallbacks - Fail-closed behavior control

Compatibility & Policy Filters:

• require_parameters - Strict parameter support
• data_collection - Allow/deny data collection
• zdr - Zero Data Retention endpoints only

Performance & Cost Preferences:

• sort - By price/throughput/latency (supports complex partition config for multi-model)
• preferred_min_throughput / preferred_max_latency - Simple or percentile thresholds
• max_price - Hard price ceiling (prompt/completion/request/image)

Quantization Filter:

• quantizations - Filter by int4/int8/fp8/fp16/bf16/fp32

let prefs = ProviderPreferences::new()
    .order(["DeepInfra", "DeepSeek"])
    .sort(ProviderSortStrategy::Throughput)
    .quantizations([Quantization::Fp8]);

let agent = client
    .agent("deepseek/deepseek-v3.2")
    .additional_params(prefs.to_json())
    .build();

Examples in: rig/rig-core/examples/openrouter_provider_selection.rs

Type of change

• New feature

Testing

• 38 unit tests covering serialization, deserialization, builder patterns, defaults, and edge cases
• Example file (openrouter_provider_selection.rs) demonstrating all features with deepseek/deepseek-v3.2
• Manually run the ‎rig/rig-core/examples/openrouter_provider_selection.rs

[joshua-mo-143]

Looks OK to me. Needs cargo fmt to pass ci

[milutinke]

@joshua-mo-143 Hello, I've fixed the issue, the checks pass now.

[milutinke]

Can we get this merged?

[joshua-mo-143]

Can we get this merged?

Hey, apologies for late response. I'll be merging today as it'll be release day today so anything that is ready-ish to be merged will be merged in 😄