[Feat]: Standardize on `builder()` method for a2a API

[jmesnil]

Is your feature request related to a problem? Please describe.

We have a mix of builder API in the a2a-spec module:

Foo foo = new Foo.Builder()
  [...]
  .build();

and

Foo foo = Foo.builder()
  [...]
  .build();

We could standardise on builder() convention and deprecate the Builder() constructor to unify the API.

see #484 (comment) for the initial discussion about this feature

Describe the solution you'd like

Provide a single builder API convention across the a2a-spec API

Describe alternatives you've considered

No response

Additional context

No response

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