Add fallback-mode option for subset

[takanabe]

Backgroud

backport v2 merged code from #1308

Critical change

This PR also change subset arguments type hints from Optional[PercentageType] to Optional[float]

We had the type error: https://github.com/cloudbees-oss/smart-tests-cli/actions/runs/27386061774/job/80933216605

Run pipenv run type
launchable/commands/subset.py:621: error: Argument 1 to "float" has incompatible type "PercentageType"; expected "Union[SupportsFloat, SupportsIndex, str, Buffer]"  [arg-type]

Click's type system works like this:

1. User runs: launchable subset --target 50%
2. Click receives the raw string "50%"
3. Click sees type=PERCENTAGE on the option, so it calls PercentageType.convert("50%", ...)
4. convert() parses "50%" and returns 0.5
5. Click passes 0.5 as target to subset()

PercentageType is called a "converter" because it converts the raw CLI string into a Python value. It's Click's extension point for custom argument types, which similar to how type=int makes Click convert "5" to 5. PercentageType does the same for percentages. The key point: convert() runs inside Click's internals, not in your code. Your function only ever sees the return value of convert(), which is a float. PercentageType itself never appears as a value anywhere in your code. This is why Optional[PercentageType] is misleading. It suggests the function receives a PercentageType instance, but it actually receives whatever convert() returned, which is float.