Support uv package manager for Python functions

Cloud Run added 3.14 support in December 2025:
https://docs.cloud.google.com/run/docs/release-notes
https://docs.cloud.google.com/run/docs/runtimes/python

uv is also now preferred way with 3.14.

### [REQUIRED] Environment info

**firebase-tools:** 15.14.0

**Platform:** macOS

### [REQUIRED] Test case

firebase.json with python314 runtime:
```
{
"functions": [
{
"source": "functions",
"location": "europe-west4",
"codebase": "default",
"runtime": "python314"
}
],
}

```

### [REQUIRED] Steps to reproduce

firebase deploy --only functions

### [REQUIRED] Expected behavior

functions deployment works, including support for uv.

### [REQUIRED] Actual behavior
```
> firebase deploy --only functions

=== Deploying to 'redacted'...

i deploying functions
i functions: preparing codebase default for deployment
i functions: ensuring required API cloudfunctions.googleapis.com is enabled...
i functions: ensuring required API cloudbuild.googleapis.com is enabled...
i artifactregistry: ensuring required API artifactregistry.googleapis.com is enabled...

Error: Functions codebase default has invalid runtime python314 specified in firebase.json. Valid values are:
- nodejs20
- nodejs22
- nodejs24
- python310
- python311
- python312
- python313
- dart3
```