fix: resolve DeepSeek tool_calls validation errors (production ready)

After extensive systematic debugging, identified and fixed LangChain bug
where parse_tool_call() returns string args instead of dict.

**Root Cause:**
LangChain's parse_tool_call() has intermittent bug returning unparsed
JSON string for 'args' field instead of dict object, violating AIMessage
Pydantic schema.

**Solution:**
ToolCallArgsParsingWrapper provides two-layer fix:
1. Patches parse_tool_call() to detect string args and parse to dict
2. Normalizes non-standard tool_call formats to OpenAI standard

**Implementation:**
- Patches parse_tool_call in langchain_openai.chat_models.base namespace
- Defensive approach: only acts when string args detected
- Handles edge cases: invalid JSON, non-standard formats, invalid_tool_calls
- Minimal performance impact: lightweight type checks
- Thread-safe: patches apply at wrapper initialization

**Testing:**
- Confirmed fix working in production with DeepSeek Chat v3.1
- All tool calls now process successfully without validation errors
- No impact on other AI providers (OpenAI, Anthropic, etc.)

**Impact:**
- Enables DeepSeek models via OpenRouter
- Maintains backward compatibility
- Future-proof against similar issues from other providers

Closes systematic debugging investigation that spanned 6 alpha releases.

Fixes: tool_calls.0.args validation error [type=dict_type, input_type=str]

CHANGELOG.md

@@ -9,10 +9,12 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ### Fixed
 - Fixed Pydantic validation errors when using DeepSeek models via OpenRouter
-- Root cause: DeepSeek returns tool_calls in non-standard format with `args` field directly, bypassing LangChain's `parse_tool_call()`
-- Solution: Added `ToolCallArgsParsingWrapper` that normalizes non-standard tool_call format to OpenAI standard before LangChain processing
-- Wrapper converts `{name, args, id}` → `{function: {name, arguments}, id}` format
-- Includes diagnostic logging to identify format inconsistencies across providers
+- Root cause: LangChain's `parse_tool_call()` has a bug where it sometimes returns `args` as JSON string instead of parsed dict object
+- Solution: Added `ToolCallArgsParsingWrapper` that:
+  1. Patches `parse_tool_call()` to detect and fix string args by parsing them to dict
+  2. Normalizes non-standard tool_call formats (e.g., `{name, args, id}` → `{function: {name, arguments}, id}`)
+- The wrapper is defensive and only acts when needed, ensuring compatibility with all AI providers
+- Fixes validation error: `tool_calls.0.args: Input should be a valid dictionary [type=dict_type, input_value='...', input_type=str]`
 
 ## [0.4.1] - 2025-11-06