handle unnamed lambdas for consistency

[zzstoatzz]

this PR reorders the validation to catch invalid fn name fallbacks before attempting to create the flow in the API, and renames a fn set to an unnamed lambda (which currently falls back to <lambda>) to unknown-lambda to handle this case

---

besides disallowing < and > in names of Flow objects in the API, the flow decorator just works with lambdas (though granted its not recommended) and IMO we should either handle this case or explicitly not support lambdas (which we do already use for fn in the tests)

before

» python -c "from prefect import flow; flow(log_prints=True)(lambda: print(42))()"
Traceback (most recent call last):
...
prefect.exceptions.PrefectHTTPStatusError: Client error '422 Unprocessable Entity' for url 'https://api.prefect.cloud/api/accounts/xxx/workspaces/xxx/flows/'
Response: {'exception_message': 'Invalid request received.', 'exception_detail': [{'loc': ['body', 'name'], 'msg': "Name '<lambda>' contains an invalid character. Must not contain any of: ['/', '%', '&', '>', '<'].", 'type': 'value_error.invalidname'}], 'request_body': {'name': '<lambda>', 'tags': []}}

after

» python -c "from prefect import flow; flow(log_prints=True)(lambda: print(42))()"
21:34:07.570 | INFO    | prefect.engine - Created flow run 'psychedelic-bullmastiff' for flow 'unknown-lambda'
21:34:07.861 | INFO    | Flow run 'psychedelic-bullmastiff' - 42
21:34:08.022 | INFO    | Flow run 'psychedelic-bullmastiff' - Finished in state Completed()

[codspeed-hq]

CodSpeed Performance Report

Merging #15400 will not alter performance

_{Comparing lambda (dc8de4a) with main (8f159b4)}

Summary

✅ 3 untouched benchmarks

[cicdw]

makes sense to me, dynamicism is the name of the game