fix(core): preserve structured inputs on tool runs in tracers (#37108)

Tool runs in `_TracerCore._create_tool_run` were discarding the
structured `inputs` dict that `BaseTool.run` passes to `on_tool_start`,
replacing it with `{"input": str(filtered_tool_input)}`. Consequently,
every multi-arg tool (e.g. ones in `deepagents` like `execute`,
`edit_file`, `write_file`, `grep`, ...) appeared in LangSmith with a
stringified, escaped dump of its arguments — multi-line bash commands
rendered with `\n` and were effectively unreadable. Chain runs already
preserved dicts via `_get_chain_inputs`; tool runs are now symmetric.

## Changes
- Preserve `inputs` when it is already a `dict` in the `original` /
`original+chat` branch of `_TracerCore._create_tool_run`, falling back
to `{"input": input_str}` only when no structured payload was provided
- Add regression tests in the sync and async base-tracer suites that
pass a structured `inputs` to `on_tool_start` and assert the dict
survives onto the resulting `Run`

## Breaking change
Custom `BaseTracer` subclasses that parsed `Run.inputs["input"]` as a
stringified dict for tool runs will need to read the structured fields
directly. The shape now matches what `on_tool_start(inputs=...)` has
always received — introduced alongside `_schema_format` in the
`astream_events` work — and what `streaming_events` consumers already
see.

libs/core/langchain_core/tracers/core.py

@@ -449,7 +449,7 @@ class _TracerCore(ABC):
             kwargs.update({"metadata": metadata})
 
         if self._schema_format in {"original", "original+chat"}:
-            inputs = {"input": input_str}
+            inputs = inputs if isinstance(inputs, dict) else {"input": input_str}
         elif self._schema_format == "streaming_events":
             inputs = {"input": inputs}
         else: