refactor(rust): shrink uniform API call errors

[adamcavendish]

Summary

Make generated Rust ApiCallError values pointer-sized by moving their private
operation metadata and erased error state behind one Box.

Problem

ApiCallError previously stored its largest error variant inline, so every
erased error carried the full enum storage even though typed operation errors
remain the common representation before erasure.

Behavior

Situation	Behavior
Typed operation error	Remains inline and allocation-free
Erased ApiCallError	Stores one boxed private representation
Public accessors, Display, and Error::source	Preserve their existing behavior
Derived Debug output	Reflects the new private inner wrapper

Compatibility

Public error methods and calling semantics are unchanged. Constructing an
erased ApiCallError performs one heap allocation. The derived Debug format
changes intentionally; it is not treated as a stable string-processing API.

Validation

• just fmt-check
• just clippy
• cargo test api_call_error_
• cargo test --test typed_http_error_runtime
• just golden-check
• just golden-rust::build (63/63 reqwest, 63/63 ureq, 63/63 aioduct)
• cargo test --doc --workspace
• just test-all (1303/1304 passed locally; the Kotlin charset smoke test was
  blocked by the local Kotlin compiler rejecting Java 26.0.2)

[codecov]

Codecov Report

❌ Patch coverage is 82.35294% with 12 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
src/generators/rust/common/runtime.rs	82.35%	12 Missing ⚠️

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