Fix/anthropic stream tool blocks

[abdalla1912mohamed]

Fix Anthropic 400: Merge consecutive tool_result messages for API compliance

Summary

Fixes Anthropic API 400 errors (unexpected tool_use_id found in tool_result blocks) when an assistant turn includes multiple tool calls. Anthropic requires all tool results for a given assistant turn to be in a single user message. The client was sending separate Tool messages (one per result), which produced separate user messages and violated that rule.

Problem

When using direct Anthropic (ANTHROPIC_API_KEY), multi-tool-call turns failed with:

Anthropic API error 400: messages.N.content.0: unexpected `tool_use_id` found in `tool_result` blocks. 
Each `tool_result` block must have a corresponding `tool_use` block in the previous message.

Root cause: Separate tool-result messages make the 2nd+ tool_result reference a user message (the previous tool result) instead of the assistant message with tool_use blocks.

Before (broken)	After (fixed)
assistant (tool_use A, B)	assistant (tool_use A, B)
user (tool_result A)	user (tool_result A, tool_result B)
user (tool_result B) ← 400	← single merged message

Changes

• libs/ai/src/providers/anthropic/convert.rs
  • Added merge_consecutive_tool_messages() to merge consecutive Role::Tool messages into one before converting to Anthropic format.
  • Wired into build_messages_with_caching() so it runs for all requests.
  • Added test_consecutive_tool_messages_merged_for_anthropic to cover merged format.

Testing

• cargo test -p stakai test_consecutive_tool_messages_merged
• Manual: Use ANTHROPIC_API_KEY with a prompt that triggers multiple tool calls in one turn; confirm no 400 errors.

Scope

• Direct Anthropic only. When using STAKPAK_API_KEY, requests go through Stakpak’s backend; the merge must be implemented there. This PR does not fix the Stakpak cloud path.
• Single-tool turns are unchanged; merging is effectively a no-op for one tool result.

References

• Anthropic Tool Use

[ahmedhesham6]

We don't want to change the stakai layer, cause its not its problem it should be in the AgentLoop