fix: align HTTP+JSON REST interop

[muscariello]

Summary

Align the Rust HTTP+JSON binding with the A2A 1.0 and cross-SDK REST contract.

• switch the Rust REST client to canonical HTTP+JSON routes such as /message:send, /tasks/{id}:cancel, /tasks/{id}:subscribe, /tasks/{id}/pushNotificationConfigs, and /extendedAgentCard
• preserve structured A2A error codes when the Rust REST client receives non-2xx responses
• serve canonical HTTP+JSON routes from the Rust REST server and emit structured google.rpc.Status-style error payloads with ErrorInfo details
• keep the older slash-style REST routes on the server as compatibility aliases while clients migrate
• add regression coverage for REST error parsing and end-to-end transport behavior

Validation

• cargo fmt --all
• cargo test --workspace

Context

This is the upstream fix needed to convert the CSIT Rust <-> Go REST legs from documented incompatibilities into true interoperability passes.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 82.07547% with 57 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
a2a-server/src/rest.rs	81.54%	31 Missing ⚠️
a2a-client/src/rest.rs	82.66%	26 Missing ⚠️

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