feat(cli): migrate flow command to cyclopts (wave 3a)

[zzstoatzz]

Native cyclopts implementation of flow ls and flow serve, replacing the delegation stub that forwarded to typer.

What changed

• New: src/prefect/cli/_cyclopts/flow.py — native ls and serve commands with deferred imports
• Wiring: replaced delegated stub in _cyclopts/__init__.py with native import, removed "flow" from _DELEGATED_COMMANDS, added to _CYCLOPTS_COMMANDS
• Tests: mock_runner_start fixture patches Runner.start on the class directly (works for both import paths); test_flow_serve_cli_requires_entrypoint handles different framework error messages/codes
• Gotcha: flow_app uses version_flags=[] to prevent cyclopts from intercepting --version 1.0.0 in flow serve

Test plan

• uv run pytest tests/cli/test_flow.py — 7 passed (typer)
• PREFECT_CLI_FAST=1 uv run pytest tests/cli/test_flow.py — 7 passed (cyclopts)
• uv run pytest tests/cli/test_cyclopts_parity.py — 18 passed
• ruff clean

🤖 Generated with Claude Code

[codspeed-hq]

Merging this PR will not alter performance

✅ 2 untouched benchmarks

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_{Comparing cli-cyclopts/wave-3a (43a7ad0) with main (5b81df1)}