[Bug]: Some optional properties are required

### What happened?

The `filename` property of the `Part` interface is clearly documented as optional, yet it's a required property, which doesn't even accept `undefined`. Within the same interface, there's also `metadata`, which is again documented as optional, but it's in fact a required property (although in this case at least it does accept undefined). In fact, according to [the spec for Part,](https://a2a-protocol.org/v1.0.1/specification/#416-part) **ALL** properties should be optional.

There are many other similar cases, for example in the `Message` interface, where `contextId`, `taskId` and `metadata` are all documented as optional, yet required in code.

I'm aware the code is generated from the `a2a.proto` from the spec repo, but that's also the case for the Python SDK, for example, which doesn't have this issue.

On top of that, I don't understand the need to use `Part.content` as a wrapper over the actual content of the part. Why not use `Part.text` / `raw` / etc, as is done in the Python library?

EDIT:
There are also examples of the opposite kind of problem, having properties which should always be required as possibly undefined, like in the case of `TaskArtifactUpdateEvent.artifact`

### Relevant log output

```shell

```

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