fix: Remove unimplemented state_transition_history capability field (#1396)

Removes the `state_transition_history` field from `AgentCapabilities`
message as it has no corresponding API implementation. This capability
flag could mislead clients into expecting functionality that doesn't
exist in v1.0.

The field can be reintroduced in a future version when a proper
implementation is designed, including:
- Adding status_history field to Task message
- Adding include_status_history parameters to Get/List operations
- Defining behavior for history storage and retrieval

Fixes #1228

Co-authored-by: madankumarpichamuthu <madankumarpichamuthu@users.noreply.github.com>
Co-authored-by: Luca Muscariello <muscariello@ieee.org>

Author: madankumarpichamuthu

specification/grpc/a2a.proto

@@ -480,8 +480,8 @@ message AgentCapabilities {
   optional bool push_notifications = 2;
   // A list of protocol extensions supported by the agent.
   repeated AgentExtension extensions = 3;
-  // Indicates if the agent provides a history of state transitions for a task.
-  optional bool state_transition_history = 4;
+  // Field 4 was previously used by state_transition_history capability.
+  reserved 4;
   // Indicates if the agent supports providing an extended agent card when authenticated.
   optional bool extended_agent_card = 5;
 }