feat: implement an option for storing file parts as files

### Which area does this relate to?

- [x] CLI code
- [ ] SPEC or COMPLIANCE file
- [ ] Other doc files

### What kind of feature are you proposing?

New command or flag

### What would you like to see?

Add a flag `--save-fileparts ` or similar that if passed will instruct the CLI to look for filename in response Artifact and Message parts and store them under the provided directory as files.
If a filename doesn't have an extension we can try inferring it from mediaType.

The proposal is to append bytes to a file while they correspond to the same artifact. If the same filename is returned for a different artifact it's likely the agent is regenerating a response, so it's better to save it with (2) / (3) / (n) in the name if a file already exists.

### Why is this important?

Raw bytes printed to the terminal are hard to deal with.

### Who benefits from this feature?

a2a cli users, especially when connecting to media-producing agents.

### Would you be interested in helping build this feature?

Yes — I'd like to implement it

### Additional context

_No response_

### Code of Conduct

- [x] I agree to follow this project's Code of Conduct