[DNM] Only send the delta on the partial tool call

[rumpl]

Do not merge, only opening to see if there are any test failures, some things fail for me locally

[docker-agent]

Review Summary

Assessment: 🔴 CRITICAL

This PR introduces a critical bug in the partial tool call event merging logic that will cause function names to be lost during streaming.

Critical Issues Found

1 HIGH severity bug that will break streaming tool call displays
1 MEDIUM severity bug causing inconsistent event metadata

Details

The changes implement delta-based streaming for tool call arguments (sending only new tokens instead of the full accumulated string), but the merging logic in pkg/app/app.go has a critical flaw that will lose function names when consecutive events are merged.

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Findings

[docker-agent]

🔴 HIGH: Function name lost during consecutive PartialToolCallEvent merges

The cmp.Or(next.ToolCall.Function.Name, latest.ToolCall.Function.Name) logic is incorrect for merging delta events.

The Problem:

• The first event ev contains the full function name (accumulated from streaming.go)
• Subsequent events next have empty names because they're just argument deltas
• cmp.Or returns the first non-empty value, preferring next.ToolCall.Function.Name (empty) over latest.ToolCall.Function.Name
• This causes the function name to be lost after the first merge iteration

The Fix:
Use ev.ToolCall.Function.Name (from the first event in the merge chain) instead:

Name: ev.ToolCall.Function.Name,

This preserves the name learned from the first event throughout all subsequent merges.

[docker-agent]

🟡 MEDIUM: Inconsistent AgentContext and ToolDefinition sources in merged event

The merged event uses AgentContext from the first event but ToolDefinition from the last event, creating an inconsistency.

The Issue:

AgentContext:   ev.AgentContext,      // from first event
ToolDefinition: next.ToolDefinition,  // from last event

This means the timestamp/context metadata comes from when the tool call was initiated, but the tool definition comes from the last delta event. While tool definitions are unlikely to change mid-stream, this violates consistency and could cause subtle bugs if tool definitions are updated dynamically.

Recommendation:
For consistency, use the tool definition from the first event:

ToolDefinition: ev.ToolDefinition,