fix: normalize DeepSeek non-standard tool_calls format

Systematic debugging revealed DeepSeek returns tool_calls in non-standard
format that bypasses LangChain's parse_tool_call():

**Root Cause:**
- OpenAI standard: {function: {name, arguments}, id}
- DeepSeek format: {name, args, id}
- LangChain's parse_tool_call() returns None when no 'function' key
- Result: Raw tool_call with string args → Pydantic validation error

**Solution:**
- ToolCallArgsParsingWrapper detects non-standard format
- Normalizes to OpenAI standard before LangChain processing
- Converts {name, args, id} → {function: {name, arguments}, id}
- Added diagnostic logging to identify format variations

**Impact:**
- DeepSeek models now work via OpenRouter
- No breaking changes to other providers (defensive design)
- Diagnostic logs help debug future format issues

Fixes validation errors:
  tool_calls.0.args: Input should be a valid dictionary
  [type=dict_type, input_value='{"symbol": "GILD", ...}', input_type=str]

CHANGELOG.md

@@ -7,6 +7,13 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+### 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
+
 ## [0.4.1] - 2025-11-06
 
 ### Fixed