fix: expose HTTP status code in JSONRPCTransport errors

[pratik3558]

Previously, when the JSONRPC transport received a non-2xx HTTP response, it threw a plain IOException with the status code embedded as a string (e.g. "Request failed 503"). This made it impossible for callers to programmatically distinguish 4xx from 5xx errors without fragile string parsing.

This change makes the structured HTTP status code available while remaining fully backward compatible:

• JSONRPCTransport.sendPostRequest now throws A2AClientException with an A2AClientHTTPError as the cause, instead of a plain IOException. All transport methods already declared 'throws A2AClientException' and already had 'catch (A2AClientException e) { throw e; }', so no caller signatures change.

• A2AClientHTTPError gains a new 'responseBody' field (and getResponseBody() accessor) so callers can also inspect the raw error payload returned by the server (e.g. a gateway's JSON error body on a 429 or 503). The existing 'code', 'message', and getCode() are preserved unchanged. The old Object-typed constructor is deprecated in favour of the new String-typed one that actually stores the body.

Callers that want the status code can now opt in with a simple instanceof check, while existing catch blocks for A2AClientException continue to work without modification:

} catch (A2AClientException e) {
    if (e.getCause() instanceof A2AClientHTTPError httpErr) {
        int status = httpErr.getCode();          // e.g. 503
        String body = httpErr.getResponseBody(); // raw response body
    }
}

Description

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Fixes #799 🦕

[gemini-code-assist]

Code Review

This pull request enhances HTTP error handling by introducing A2AClientHTTPError as a structured cause within A2AClientException, allowing callers to retrieve the HTTP status code and response body. Feedback highlights a missing throws declaration for A2AClientException in the sendPostRequest method, which would lead to a compilation error, and suggests refactoring duplicated error message strings for better maintainability.