feat: add rerank endpoint support

[tbille]

Description

Add rerank support to the Rust SDK, allowing users to rerank documents by relevance to a query via the gateway provider. This is the first non-completion, non-moderation endpoint and follows the established SUPPORTS_STREAMING / SUPPORTS_TOOLS trait extension pattern.

Changes:

• New RerankParams, RerankResult, RerankMeta, RerankUsage, and RerankResponse types in src/types/rerank.rs
• SUPPORTS_RERANK associated constant and rerank()/rerank_fn() two-level dispatch methods on the Provider trait
• Gateway provider implementation via reqwest POST to /v1/rerank, reusing convert_error() for HTTP error mapping
• GatewayRerankRequest wire type that maps model_id to model (consistent with completions)
• Public rerank() free function and RerankOptions in src/api.rs
• Re-exports in src/lib.rs

PR Type

• 🆕 New Feature

Relevant issues

Checklist

• I understand the code I am submitting.
• I have added unit tests that prove my fix/feature works
• I have run this code locally and verified it fixes the issue.
• New and existing tests pass locally (cargo test --all-features)
• Code passes linting (cargo clippy --all-features -- -D warnings)
• Code is formatted (cargo fmt)
• Documentation was updated where necessary
• I have read and followed the contribution guidelines
• AI Usage:
  • No AI was used.
  • AI was used for drafting/refactoring.
  • This is fully AI-generated.

AI Usage Information

• AI Model used: Claude claude-opus-4-6
• AI Developer Tool used: OpenCode
• Any other info you'd like to share: Implementation follows the spec in any-llm-rust-spec.md for adding rerank endpoint support.