fix(cli): keep stdout valid JSON when SDK calls print in --json mode

[cdeil]

Summary

Fixes #546.

• roboflow/cli/_output.py: new context manager sdk_output_to_stderr(args). With --json it redirects stdout to stderr; without --json it does nothing.
• search --export, image upload <dir>, model upload and version download run their SDK call inside it, so the SDK's progress and status lines move to stderr and stdout carries only the JSON result.
• model upload now wraps the workspace init in suppress_sdk_output() like the other handlers, which drops "loading Roboflow workspace..." from its output.
• Text mode (without --json) is unchanged.

Why stderr and not suppress_sdk_output(): for image upload <dir>, the per-image [ERR] lines are currently the only failure signal (#545), so they should stay visible. In JSON mode, stderr already carries the tqdm progress bars.

Test plan

• tests/cli/test_json_stdout.py: one test per handler with an SDK mock that prints, asserting that stdout parses with json.loads and that the printed line ends up on stderr, plus one test that text mode keeps SDK output on stdout. The four handler tests fail on main and pass with this change.
• End to end, with HTTP mocked by responses and the commands run through typer.testing.CliRunner: all four commands now print only the JSON document on stdout.
• Python 3.13: make check_code_quality passes; python -m unittest runs 1214 tests, OK (1 skipped).