feat: machine-readable JSON error output for -o json

[msampathkumar]

Goal: Make a2a failures machine-readable under -o json, so a program can handle an error the same way it handles a result.
User story: As a program or agent consuming -o json, I want failures returned as a JSON object just like successes, so that I can detect and branch on errors with a single JSON parse instead of scraping stderr.

Priority: P0 · Area: CLI code

Problem

On failure the tool prints a human-readable Error: <text> line to stderr — even when the caller asked for -o json. A program gets a parseable object on success but an unparseable plaintext line on failure.

$ a2a card get http://127.0.0.1:9999 -o json     # nothing listening
Error: failed to resolve agent card: ... connect: connection refused

$ a2a send --bogus -o json 'x'
Error: unknown flag: --bogus

Proposed behavior

Under -o json, a failure emits exactly one error object on stdout (diagnostics stay on stderr):

{
  "error": {
    "code":    "string",
    "message": "human-readable string",
    "hint":    "actionable next step, or null",
    "a2aCode": "underlying transport code, or null"
  }
}

• code — a protocol error carries the agent's error name (e.g. TaskNotFoundError); a CLI-local condition carries a symbolic A2ACLI_ERR_* code.
• Proposed CLI-local codes: A2ACLI_ERR_USAGE, A2ACLI_ERR_CARD_NOT_FOUND, A2ACLI_ERR_CARD_INVALID, A2ACLI_ERR_UNREACHABLE, A2ACLI_ERR_CREDENTIALS_MISSING, A2ACLI_ERR_AUTH_FAILED, A2ACLI_ERR_TIMEOUT, A2ACLI_ERR_INTERNAL.
• hint — a short, copy-pasteable next step when one exists, else null.

Examples:

{"error":{"code":"A2ACLI_ERR_CARD_NOT_FOUND","message":"no agent card at http://127.0.0.1:9999","hint":"check the --agent-card reference","a2aCode":null}}
{"error":{"code":"A2ACLI_ERR_USAGE","message":"unknown flag: --bogus","hint":"run: a2a send --help","a2aCode":null}}

Under --stream, an error that terminates the stream is emitted as a final error object on its own line. text mode may keep the readable Error: line.

Acceptance criteria

• -o json failures emit the error object on stdout; nothing but structured output on stdout.
• Protocol failures carry the agent's error name; CLI-local failures carry an A2ACLI_ERR_* code.
• hint populated where an actionable next step exists.
• Tests cover: unresolvable card, usage error, and a protocol error.

Environment

• CLI version: a2a version v0.0.0-…1e29dfe94f95+dirty
• OS: Linux x86_64

Notes

Pairs with the distinct-exit-codes request — both live in Execute()/error handling (internal/cli/root.go); the reported code and the exit code should agree.