feat(cli): Add trace:get CLI command

[ssbushi]

Fixes: #5379

Usage:

$ genkit trace:get 5aa367e7d3895db78339f6148aab29b4
Telemetry API running on http://localhost:4033
{
  "traceId": "5aa367e7d3895db78339f6148aab29b4",
  "displayName": "generate",
  "startTime": 1778700071212,
  "endTime": 1778700072191.0662,
....

Checklist (if applicable):

• PR title is following https://www.conventionalcommits.org/en/v1.0.0/
• Tested (manually, unit tested, etc.)

[gemini-code-assist]

Code Review

This pull request introduces a new trace:get command to the Genkit CLI, enabling users to fetch trace details by ID. The changes include the command implementation, its registration in the main CLI entry point, and a comprehensive set of unit tests covering success and error paths. I have no feedback to provide.