[Feature]: Refactor implementation to move away from A2AClient and adapt the new pattern introduced in a2a-sdk v0.3.0

[panyuenlau]

Problem Statement

The a2a-sdk's A2AClient class that observe-sdk's A2AInstrumentor relies on has been deprecated since v0.3.0: https://github.com/a2aproject/a2a-python/blob/v0.3.0/src/a2a/client/legacy.py#L40-L41.

The community has introduced a new pattern for creating client to agents:

• Client interfaec: https://github.com/a2aproject/a2a-python/blob/v0.3.0/src/a2a/client/client.py#L81-L87
• BaseClient implementation: https://github.com/a2aproject/a2a-python/blob/v0.3.0/src/a2a/client/base_client.py#L29
• ClientFactory: https://github.com/a2aproject/a2a-python/blob/v0.3.0/src/a2a/client/client_factory.py#L38-L52

Proposed Solution

We should start moving away from A2AClient and start adapting the newly introduced pattern.

Alternatives Considered

No response

Additional Context

No response

Checklist

• I have read the contributing guidelines
• I have verified this does not duplicate an existing feature request

[91pavan]

#92