Preserve structured JSON-RPC error data

[ellismg]

Summary

• preserve optional structured JSON-RPC error data on copilot::Error
• expose non-null object, array, and scalar data through Error::rpc_data(); omitted and null data return None
• preserve existing RPC error kind, numeric code, message, Display/Debug formatting, and source behavior; payload access stays explicit
• add framed JSON-RPC regressions for payload shapes, omitted/null data, and formatting compatibility

Fixes #2726

Follow-up validation

The Debug compatibility assertion failed before the correction (RED) and passed afterward (GREEN). The focused client_call_ selection passes all 3 tests; the affected error selection passes all 6 tests with backtraces enabled.

Commands run from rust/:

COPILOT_SKIP_CLI_DOWNLOAD=1 cargo test --no-default-features --features test-support --test prepared_session_test client_call_
COPILOT_SKIP_CLI_DOWNLOAD=1 RUST_BACKTRACE=1 cargo test --no-default-features --features test-support --test prepared_session_test error
COPILOT_SKIP_CLI_DOWNLOAD=1 cargo check --no-default-features --features test-support --lib --test prepared_session_test
rustup run nightly-2026-04-14 rustfmt --check --config-path .rustfmt.nightly.toml --config skip_children=true src/errors.rs src/lib.rs tests/prepared_session_test.rs

git diff --check also passed. The framed tests use in-memory transport, with runtime downloads disabled. The existing validate_runtime_pair dead-code warning in that configuration is unchanged. No full local suite or Clippy rerun was needed for the follow-up.

Original author-reported validation

• cargo test --all-features --test prepared_session_test client_call_
• rustup run nightly-2026-04-14 cargo fmt --check
• cargo clippy --all-features --all-targets -- -D warnings
• cargo test --all-features

[aurokin]

Updated the Rust error-data change to preserve existing Debug formatting. Structured data remains available through Error::rpc_data(), with focused framed-protocol coverage for JSON values and omitted/null data. Follow-up validation is recorded in the PR description.

[Copilot]

Copilot review overview

🟢 Approval recommended

The implementation meets the stated contract with focused regression coverage and no unresolved issues.

Review effort: Balanced
Findings: None

What changed in this PR

Preserves structured JSON-RPC error payloads in the Rust SDK while maintaining existing error behavior.

Changes:

• Adds explicit Error::rpc_data() access.
• Retains payloads during RPC error conversion.
• Covers payload shapes, null handling, and formatting compatibility.

File	Description
rust/​tests/​prepared_session_test.rs	Adds framed RPC error regression tests.
rust/​src/​lib.rs	Preserves data during RPC error conversion.
rust/​src/​errors.rs	Stores and exposes optional RPC data.

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