fix(nubi): support --format json machine-readable output in single query mode

[blue4209211]

Summary

This PR adds support for machine-readable JSON output (--format json) when executing nbctl nubi in single-query mode (both synchronous and asynchronous).

Changes

• Machine-Readable Output: When --format json is specified with -q / --query:
  • Suppresses TUI markdown rendering, lipgloss borders, and spinner progress output.
  • Outputs a structured JSON payload containing query, response, status, conversation/session/account IDs, execution duration, and web URL.
  • Returns structured JSON payloads for async mode (--async) and context cancellation.
• Testing: Added TestNubiCmd_SyncQuery_JSON unit test in cmd/nubi_test.go verifying JSON payload parsing and key assertions.

Verification

• golangci-lint run ./...: 0 issues
• go test ./...: 100% pass

[gemini-code-assist]

Code Review

This pull request introduces JSON output formatting support to the nubi command, allowing users to retrieve structured execution details, and adds a corresponding unit test. The review feedback suggests enhancing this feature by ensuring all execution errors are returned in JSON format, parsing the raw usage metrics string into a structured nested JSON object for better machine readability, and adding corresponding test assertions to verify the metrics structure.