Enable lazy loading for CLI commands via cyclopts v4

[zzstoatzz]

Summary

• Upgrade cyclopts from v3 to v4 and convert all 29 command registrations to lazy string-based imports (app.command("module:attr"))
• Command modules are only imported when actually invoked, eliminating ~1s of unnecessary import overhead for targeted commands
• Fix test that relied on implicit module availability via parent package attribute access

Performance

Before (all 25+ command modules imported eagerly at import prefect.cli._app):

import prefect.cli._app: ~3300ms
CLI modules loaded at import: 25+

After (zero command modules imported until needed):

import prefect.cli._app: ~1000ms
CLI modules loaded at import: 0
Resolving single command (e.g. 'config'): ~1ms

Note: prefect --help still imports all modules (cyclopts resolves all lazy commands to render the help page). The win is for prefect <specific-command> which now only imports the module it needs.

Test plan

• All CLI tests pass (1152 passed, known-flaky start_server tests excluded)
• All command aliases verified (flows, profiles, blocks, gcl, etc.)
• Smoke-tested: --help, version, config view, profile ls, flow-run ls, server services ls, deploy --help
• Verified zero command modules loaded at import time
• Pre-commit hooks pass

🤖 Generated with Claude Code

[codspeed-hq]

Merging this PR will not alter performance

✅ 2 untouched benchmarks

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_{Comparing cli-lazy-loading (d09cce7) with main (15781ee)}