Add MessageDelta streaming tool-call status helpers

[brainlid]

Problem

Streaming consumers (Phoenix LiveView, Sagents) need to track the lifecycle of a tool call as it arrives in pieces across multiple MessageDelta events: first the call is identified, then display_text is filled in, then execution status moves through executing → completed/failed. Today every consumer reimplements the same merge-and-update logic against delta.tool_calls, including subtle rules like "never overwrite an existing display_text with nil" and "don't clear the in-flight UI until every sibling tool call has terminated." Those rules belong on the library side.

Solution

Add four small, well-documented helpers to LangChain.MessageDelta that codify the merge rules:

• upsert_tool_call/2 — append a new ToolCall or merge non-nil fields onto an existing one with the same call_id. status is promoted one-way (:incomplete → :complete, never back), and metadata is shallow-merged with incoming keys winning per-key.
• set_tool_execution_status/3 — drive the metadata["execution_status"] lifecycle for a specific call_id, delegating to ToolCall.set_execution_status/2.
• set_tool_display_text/3 — refine display_text as more is learned ("Reading file" → "Reading \"outline.md\" (lines 60–100)"). A nil incoming value is an explicit no-op so the UI never regresses from showing something to showing nothing.
• all_tools_terminal?/1 — gate UI cleanup on every tool call reaching a terminal status. Returns false for nil or empty tool_calls so empty deltas don't trigger premature clears.

All helpers are additive — no existing MessageDelta behavior changes — and they tolerate nil/empty tool_calls so callers don't need to guard.

Changes

• lib/message_delta.ex — Added upsert_tool_call/2, set_tool_execution_status/3, set_tool_display_text/3, all_tools_terminal?/1, plus private merge_tool_call_fields/2, promote_status/2, merge_metadata/2, and update_matching_tool_call/3 helpers. Each public function carries a @spec and a doctest covering the typical case.
• test/message_delta_test.exs — Added four describe blocks (one per helper) covering: append vs. merge on upsert_tool_call, nil tool_calls handling, per-field merge precedence, one-way :complete status promotion, shallow metadata merge, set_* no-ops when no matching call_id is present, the nil-display-text no-op rule, and terminal/non-terminal cases for all_tools_terminal?.

Testing

• New unit tests in test/message_delta_test.exs cover both happy paths and the edge cases that motivated each rule (nil tool_calls, missing call_id, status downgrades, nil display_text).
• Doctests on each public function exercise the typical streaming usage shape.
• No live API tests required — these helpers are pure data transforms on MessageDelta.

Run locally with:

cd my_langchain && mix test test/message_delta_test.exs