Fix DeepSeek Reasoner tool-call handling and add reasoning_content support (#44301)

## Closes #43887

## Release Notes:

### Problem
DeepSeek's reasoning mode API requires `reasoning_content` to be
included in assistant messages that precede tool calls. Without it, the
API returns a 400 error:

```
Missing `reasoning_content` field in the assistant message at message index 2
```

### Added/Fixed/Improved
- Add `reasoning_content` field to `RequestMessage::Assistant` in
`crates/deepseek/src/deepseek.rs`
- Accumulate thinking content from `MessageContent::Thinking` and attach
it to the next assistant/tool-call message
- Wire reasoning content through the language model provider in
`crates/language_models/src/provider/deepseek.rs`

### Testing
- Verified with DeepSeek Reasoner model using tool calls
- Confirmed reasoning content is properly included in API requests

Fixes tool-call errors when using DeepSeek's reasoning mode.

---------

Co-authored-by: Ben Brandt <benjamin.j.brandt@gmail.com>

crates/deepseek/src/deepseek.rs

@@ -155,6 +155,8 @@ pub enum RequestMessage {
         content: Option<String>,
         #[serde(default, skip_serializing_if = "Vec::is_empty")]
         tool_calls: Vec<ToolCall>,
+        #[serde(default, skip_serializing_if = "Option::is_none")]
+        reasoning_content: Option<String>,
     },
     User {
         content: String,