chore(client): Expose BaseClient from a2a.client package (#529)

# Description

This pull request introduces a minor update to the
`src/a2a/client/__init__.py` file, focusing on the module's imports. The
change makes the `BaseClient` class available for import from this
package.

* Added `BaseClient` to the list of imported symbols and to the module's
`__all__`, making it accessible when importing from `a2a.client`.
[[1]](diffhunk://#diff-e472e7970567ab144a39be570324db52d9da69a3f678438f6ee5f3aecd79b474R10)
[[2]](diffhunk://#diff-e472e7970567ab144a39be570324db52d9da69a3f678438f6ee5f3aecd79b474R55)

---

- [x] Follow the [`CONTRIBUTING`
Guide](https://github.com/a2aproject/a2a-python/blob/main/CONTRIBUTING.md).
- [x] Make your Pull Request title in the
<https://www.conventionalcommits.org/> specification.
- Important Prefixes for
[release-please](https://github.com/googleapis/release-please):
- `fix:` which represents bug fixes, and correlates to a
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- `feat:` represents a new feature, and correlates to a SemVer minor.
- `feat!:`, or `fix!:`, `refactor!:`, etc., which represent a breaking
change (indicated by the `!`) and will result in a SemVer major.
- [x] Ensure the tests and linter pass (Run `bash scripts/format.sh`
from the repository root to format)
- [x] Appropriate docs were updated (if necessary)

Co-authored-by: Lukasz Kawka <[email protected]>

Author: martimfasantos

src/a2a/client/__init__.py

@@ -7,6 +7,7 @@
     CredentialService,
     InMemoryContextCredentialStore,
 )
+from a2a.client.base_client import BaseClient
 from a2a.client.card_resolver import A2ACardResolver
 from a2a.client.client import Client, ClientConfig, ClientEvent, Consumer
 from a2a.client.client_factory import ClientFactory, minimal_agent_card
@@ -51,6 +52,7 @@ def __init__(self, *args, **kwargs):
     'A2AClientTimeoutError',
     'A2AGrpcClient',
     'AuthInterceptor',
+    'BaseClient',
     'Client',
     'ClientCallContext',
     'ClientCallInterceptor',