fix(a2a-grpc): preserve precise A2A error codes

[jstar0]

Summary

The gRPC boundary currently maps errors using only tonic::Code. Six distinct
A2A errors share FAILED_PRECONDITION, so clients decode all of them as
TaskNotCancelable. The server conversion also omits the
google.rpc.ErrorInfo detail that other SDKs need to recover the precise A2A
reason.

Changes

• Emit an AIP-193 ErrorInfo with the existing A2A reason and
  a2a-protocol.org domain from the server conversion.
• Decode only known reasons from that protocol domain on the client conversion.
• Preserve string metadata from existing ErrorInfo details and field
  violations from existing BadRequest details when constructing the gRPC
  status.
• Keep the current gRPC-code fallback for peers that omit details or send
  malformed, unknown, or foreign-domain details.
• Cover all six FAILED_PRECONDITION mappings in both directions, including an
  independently constructed peer status, and cover all four
  INVALID_ARGUMENT reasons plus structured detail propagation.

This uses tonic-types, the official rich error model companion for the
workspace's existing Tonic 0.14 dependency.

Compatibility

The gRPC status codes and messages are unchanged. Existing peers without rich
details continue to use the previous fallback behavior. Malformed detail
entries are ignored rather than changing status conversion.

Verification

cargo test -p a2a-grpc errors
cargo hack check --workspace --all-targets --each-feature
cargo fmt --all -- --check
cargo hack clippy --workspace --all-targets --each-feature -- -D warnings
cargo test --workspace

Fixes #103

[msardara]

Thanks for the PR @jstar0! PR looks good, but I added some comments regarding some information we potentially miss when constructing the grpc status.

[codecov]

Codecov Report

❌ Patch coverage is 99.31973% with 1 line in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
a2a-grpc/src/errors.rs	99.31%	1 Missing ⚠️

📢 Thoughts on this report? Let us know!

[msardara]

LGTM, thanks @jstar0!