Add SDK error type for custom error propagation

[manish-raana]

Fixes #11.

This change adds a public openapi_sdk::Error type and openapi_sdk::Result alias so downstream projects can integrate SDK failures into their own error enums with From<openapi_sdk::Error> and
continue using ? in existing Result pipelines.

Changes

• Added openapi_sdk::Error with variants for HTTP errors, invalid headers, and invalid HTTP methods.
• Implemented Display, std::error::Error, and From conversions for wrapped error types.
• Replaced panic-prone unwrap() calls for header and method parsing with returned SDK errors.
• Added error_for_status() handling for non-2xx HTTP responses.
• Added regression tests for custom error propagation.
• Documented custom error integration in README and crates.io docs.

Verification

• cargo fmt --all -- --check
• cargo test
• cargo test --examples