Merge "tool_call" and "tool_call_update" sessionUpdate events

[anna239]

Currently the only difference is that tool_call requires a title
Instead, specification should say that agent MUST provide a title on the first event about tool call

Compatibility

• Zed currently doesn't require a first "tool_call" event to present a tool call
• New clients can accept both "tool_call" and "tool_call_update" events and treat them equally

[benbrandt]

Yeah I think we can simplify here. I don't think we can drop either method, but maybe we can at least have them be aliases?
i.e. both take the same body? and have a single handler on the SDK side?

The only issue is that clients currently see tool_call as "replace all fields" and tool call update as "replace fields that are present"
So maybe we encourage people to just call "tool_call_update" and as a breaking change for v2 we unify them?

[anna239]

This makes sense, let's do that. Do you have some brunch for v2 already?