feat: add provider tool call lifecycle events

[TheCodingCvrlo]

Tracking issue: #6984

Summary

Surfaces provider-executed (server-side) tool calls — e.g. Mistral web search, and applicable to any provider with built-in tools — as a first-class lifecycle on AgentSession, parallel to tool_execution_updated for locally-executed tools.

• Core (llm): new ProviderToolCall payload + a provider_tool_call event on the base LLM EventEmitter (the same seam plugins already use for metrics_collected). No change to ChatChunk.
• Core (voice): new provider_tool_execution_updated session event carrying a discriminated ProviderToolCallStarted | ProviderToolCallEnded. AgentActivity bridges the LLM event onto the session, wired right next to metrics_collected.
• Plugin (mistralai): emits provider_tool_call by translating the Conversations stream's ToolExecutionStarted / ToolExecutionDone. No bespoke events on the LLM.

Mistral stream (ToolExecutionStarted / …Done)
  → plugin:    llm.emit("provider_tool_call", ProviderToolCall(phase=...))
  → framework: bridge in AgentActivity (like metrics_collected)
  → session:   emit("provider_tool_execution_updated", ProviderToolCallStarted | ProviderToolCallEnded)
  → app:       session.on("provider_tool_execution_updated", ...)

Motivation

Context: Related discussion on #5869 with @tinalenguyen and @jeanprbt . #5869 adds a single terminal function_tools_called event (once per turn). This PR gives a higher abstraction by adding the lifecycle needed to modify state while provider tools are executed, e.g. playing a "thinking"/background sound for exactly the duration of a web search.

The design mirrors the existing local-tool lifecycle (ToolCallStarted / ToolCallEnded via tool_execution_updated) so consumers get a consistent, provider-agnostic API and subscribe once.

Design notes

• Plugins only translate (provider-specific → generic ProviderToolCall) and emit on the LLM; the framework owns the app-facing provider_tool_execution_updated contract and the bridge, so any future provider plugin gets the session event for free.
• ProviderToolCallEnded carries result (the tool output) when the provider returns one.

Tests

• tests/test_plugin_mistralai_llm.py — the plugin's _parse_event emits provider_tool_call (started → delta accumulates → done with full args + result; done-without-start is safe).
• tests/test_provider_tool_events.py — end-to-end through a real AgentSession: provider_tool_execution_updated fires start→end per tool, in order.
• make check clean locally (ruff + mypy strict).

Open questions for maintainers

• Relationship to (core): add function_tools_called event #5869 — happy to align or converge (e.g. keep function_tools_called as the per-turn summary and this as the lifecycle, or unify naming).
• Scope — this wires the pipeline path; the realtime path and the remote_session protobuf relay (which would need a livekit/protocol message) are natural follow-ups.
• Naming / shape — event names, and whether result should be structured rather than str.

Opening as a draft for discussion.

[devin-ai-integration]

✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no bugs or issues to report.