Artifact metadata not synced on append operations — inconsistent state between server and client

[KlementMultiverse]

Problem

When appending data to an artifact with append=True, the artifact's metadata fields (size, updated_at, checksum) are not recalculated or returned to the client. This creates an inconsistency: the server-side artifact has been modified, but the client's in-memory representation still reflects the pre-append state.

Impact

This breaks several workflows:

1. State divergence: Client code can't trust artifact.size or artifact.updated_at after an append
2. Resumable uploads: Code that chains multiple appends and checks size/checksum between operations will have stale data
3. Consistency with other operations: Other mutations (delete, download) properly refresh metadata — append is inconsistent

Current Behavior

In src/a2a/client/task.py, the append_artifact() method likely:

• Sends the append request to the server
• Returns success/failure
• Does NOT re-fetch artifact metadata

Expected Behavior

After a successful append, the returned artifact object (or the client's cached copy) should have updated:

• size: new total size
• updated_at: current timestamp
• checksum: recalculated hash (if applicable)

Alternatively, the method should explicitly document that metadata is stale and recommend a refresh step.

Proposed Solution

One of:

1. Refresh automatically: After append succeeds, fetch and merge the updated metadata back into the artifact object
2. Return updated metadata: Have the append API response include updated metadata fields
3. Add explicit refresh method: Provide artifact.refresh_metadata() so callers can explicitly re-sync

See #735 for original report — this is the core architectural question that needs resolution before fixing the bug.

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Contributed by Klement Gunndu