feat(rust): add uniform API call errors

[adamcavendish]

Summary

Add a common ApiCallError to generated Rust clients that contain operations while preserving the existing operation-specific typed error enums. Generated conversions allow multi-operation workflows to use direct ? propagation without requiring anyhow.

Problem

Operation-specific errors preserve precise response payload types, but a workflow that calls several generated operations cannot name one ordinary error type. Applications using thiserror or SNAFU otherwise need to enumerate every generated error or map every call manually.

Behavior

Situation	Behavior
Caller needs a documented response payload	Match the existing operation-specific error enum and inspect its typed ApiError<T> body
Workflow calls multiple operations	Return Result<_, ApiCallError> and propagate each operation error directly with ?
HTTP error is converted	Preserve operation ID, status, backend-native headers, and raw response bytes
Transport or body decoding fails	Preserve the underlying error through std::error::Error::source()
Application uses thiserror or SNAFU	Use one generic application-level conversion instead of listing every operation error
OpenAPI document contains no operations	Omit ApiCallError; model-only clients remain warning-clean

Compatibility

This is additive for operation-bearing rust-reqwest, rust-ureq, and rust-aioduct clients: generated operation signatures and typed error enums remain unchanged. Converting into ApiCallError consumes the operation error and erases its decoded payload type, so callers that need that payload should match it before conversion. Generated clients do not depend on thiserror or SNAFU; those integrations are documentation and compile-test coverage only. No dead-code lint allowance is emitted.

Validation

• just fmt-check
• just clippy
• just test-all — 1,302 tests passed
• just golden-check
• RUSTFLAGS=-Dwarnings just golden-rust::build-reqwest — 63/63 clients compiled
• RUSTFLAGS=-Dwarnings just golden-rust::build-ureq — 63/63 clients compiled
• RUSTFLAGS=-Dwarnings just golden-rust::build-aioduct — 63/63 clients compiled
• just book

[codecov]

Codecov Report

❌ Patch coverage is 97.28682% with 7 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
src/generators/rust/common/runtime.rs	96.15%	6 Missing ⚠️
src/generators/rust/common/emit_api.rs	98.18%	1 Missing ⚠️

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