chore: Improve --json support accross multiple commands

[blainekasten]

Summary

Standardizes CLI JSON output on console.print_json(openapi_dumps(...).decode("utf-8")) so serialization matches the SDK (datetime, Pydantic, etc.) and stdout stays valid JSON for tooling like jq.

Changes

• Consistency: Replaced ad hoc json.dumps, rich.print_json, and raw JSON strings with the shared openapi_dumps + console.print_json pattern across touched commands (files upload/check paths, fine-tuning list/cancel, endpoints & storage retrieve, evals retrieve JSON path, jig helpers).
• Telemetry: telemetry status, enable, and disable accept the global --json flag and emit single JSON objects (telemetry status includes structured disabled/enabled + reason where relevant).
• files retrieve-content: --json emits jq-friendly payloads: stdout-only (content or content_base64), output-only (id + path), or combined {content|content_base64, path} when both --stdout and --output are set so we never print two unrelated JSON documents on one invocation.
• Jig: _print_cli_result and detach deploy summary use openapi_dumps; beta jig volumes describe prints human-readable fields without --json and full JSON with --json (replacing always-JSON behavior for the default path).
• Tests: Relaxed retrieve-content stdout assertions for optional leading newline from the loading spinner; added test_telemetry_json_mode_pipes_to_jq.

Notes

• openapi_dumps in _utils/_json.py remains the single JSON serializer for CLI output (no parallel “custom date” path).
• Subprocess JSON/jq tests that hit a mock API base URL still require that environment in CI.

Closes DX-368 (or link the Linear issue).