Transform JSON schemas for Google AI tools to use `nullable: true`
instead of `type: ["type", "null"]`, which is not supported by the
Gemini API.

Additionally, convert multi-type arrays (e.g., `type: ["string",
"number"]`)
to `anyOf` constraints, as Gemini expects a single string for the `type`
field.

This handles recursive transformation of properties, items, definitions,
and logical operators, safely merging conflicting `anyOf` and `allOf`
constraints.

Closes https://github.com/zed-industries/zed/issues/44875
Closes https://github.com/zed-industries/zed/issues/32429

Release Notes:

- Fixed a bug where using Gemini with certain tools (especially via MCP)
resulted in "Invalid JSON payload received" errors due to incompatible
JSON schema formats.

## Testing

Added unit tests in `crates/language_model/src/tool_schema.rs` covering
nullability, multi-types, and `oneOf` conversions.

### Manual Testing with MCP Test Server
The [MCP Test Server](https://github.com/dastrobu/mcp-test-server) was
used to verify these edge cases with Gemini 3 Flash.

#### Setup
1. Install the test server: `cargo install --git
https://github.com/dastrobu/mcp-test-server`
2. Add to Zed `settings.json`:
   ```json
   "context_servers": [
     {
       "command": "mcp-test-server"
     }
   ]
   ```


Use the following pattern in a chat window:

```
call the add_tool function to create a new tool: weather with input schema: 

  {
    "type": "object",
    "properties": {
      "city": { "type": ["string", "null"] }
    }
  }
```
Afterwards:
...

```
call it
```

Without fix: 

<img width="671" height="448" alt="image"
src="https://github.com/user-attachments/assets/e8b6e7bd-d431-4a7e-8c9a-fff73d82127d"
/>

With fix:

<img width="729" height="334" alt="image"
src="https://github.com/user-attachments/assets/eb758765-4c34-4915-a8cf-dd4bfb993619"
/>

#### Cases verified manually:



**1. Nullability in properties**
- **Input:**
  ```json
  {
    "type": "object",
    "properties": {
      "city": { "type": ["string", "null"] }
    }
  }
  ```
- **Converted:**
  ```json
  {
    "type": "object",
    "properties": {
      "city": { "type": "string", "nullable": true }
    }
  }
  ```

**2. Multi-type properties**
- **Input:**
  ```json
  {
    "type": "object",
    "properties": {
      "city": { "type": ["string", "number"] }
    }
  }
  ```
- **Converted:**
  ```json
  {
    "type": "object",
    "properties": {
      "city": {
        "anyOf": [
          { "type": "string" },
          { "type": "number" }
        ]
      }
    }
  }
  ```

**3. Explicit `anyOf` with nullability**
- **Input:**
  ```json
  {
    "type": "object",
    "properties": {
      "city": {
        "anyOf": [
          { "type": "string" },
          { "type": "null" }
        ]
      }
    }
  }
  ```
- **Converted:**
  ```json
  {
    "type": "object",
    "properties": {
      "city": {
        "anyOf": [
          { "type": "string" },
          { "nullable": true }
        ]
      }
    }
  }
  ```

**4. Conflicting `anyOf` sources (Multi-type + existing `anyOf`)**
- **Input:**
  ```json
  {
    "type": "object",
    "properties": {
      "city": {
        "type": ["string", "number"],
        "anyOf": [
          { "minLength": 5 }
        ]
      }
    }
  }
  ```
- **Converted:**
  ```json
  {
    "type": "object",
    "properties": {
      "city": {
        "allOf": [
          { "anyOf": [{ "minLength": 5 }] },
          { "anyOf": [{ "type": "string" }, { "type": "number" }] }
        ]
      }
    }
  }
  ```

Co-authored-by: Richard Feldman <richard@zed.dev>