Dynamic tool calls - allow dynamic registration

Add experimental app-server support for connection-owned dynamic tool providers over both `stdio` and websocket.

Clients can now:
- register a tool with `toolProvider/register`
- unregister it with `toolProvider/unregister`

Registered tools are stored in an app-server registry, mapped back to the owning connection, and routed through the existing dynamic tool call flow. Tools with `injectIntoContext: true` are exposed to the model; tools with `injectIntoContext: false` remain callable only from `js_repl` via `codex.tool(...)`.

This also extends dynamic tool call events with `thread_id` so provider-owned tools can be invoked from newly created or resumed threads.

```mermaid
sequenceDiagram
    participant Client as Provider Client
    participant AppServer as App Server
    participant ThreadClient as Thread Client
    participant Core as Core Thread

    Client->>AppServer: toolProvider/register(tool)
    AppServer->>AppServer: store tool -> owning connection

    ThreadClient->>AppServer: thread/start
    AppServer->>Core: load thread + sync registered tools

    ThreadClient->>AppServer: turn/start
    AppServer->>Core: submit user input
    Core-->>AppServer: DynamicToolCallRequest(thread_id, tool, args, provider_owned=true)

    AppServer->>Client: item/tool/call
    Client-->>AppServer: DynamicToolCallResponse
    AppServer->>Core: submit dynamic tool response
    Core-->>AppServer: continue turn

codex-rs/app-server-protocol/schema/json/EventMsg.json

@@ -1975,6 +1975,13 @@
             "callId": {
               "type": "string"
             },
+            "providerOwned": {
+              "default": false,
+              "type": "boolean"
+            },
+            "threadId": {
+              "type": "string"
+            },
             "tool": {
               "type": "string"
             },
@@ -1992,6 +1999,7 @@
           "required": [
             "arguments",
             "callId",
+            "threadId",
             "tool",
             "turnId",
             "type"
@@ -8112,6 +8120,13 @@
         "callId": {
           "type": "string"
         },
+        "providerOwned": {
+          "default": false,
+          "type": "boolean"
+        },
+        "threadId": {
+          "type": "string"
+        },
         "tool": {
           "type": "string"
         },
@@ -8129,6 +8144,7 @@
       "required": [
         "arguments",
         "callId",
+        "threadId",
         "tool",
         "turnId",
         "type"