fix(litellm): parse DeepSeek-V3 proprietary inline tool-call tokens

[fuchun1010]

Closes #5024

Problem

DeepSeek-V3 emits tool calls using proprietary special tokens embedded in the content field:

<｜tool▁calls▁begin｜><｜tool▁call▁begin｜>function<｜tool▁sep｜>analysis_input
```json
{"work_dir_name":"..."}
```<｜tool▁call▁end｜><｜tool▁calls▁end｜>

When LiteLLM does not translate these into structured tool_calls (intermittent), ADK's fallback JSON parser finds the JSON object but rejects it because the function name (analysis_input) is embedded in the tokens (<｜tool▁sep｜>analysis_input) rather than as a name key inside the JSON payload.

Result: tool call is silently dropped and the raw tokens appear as text content.

Solution

• Added _parse_deepseek_tool_calls_from_text — detects the proprietary token format, extracts function name + arguments, and emits standard ChatCompletionMessageToolCall objects
• Added _extract_json_from_deepseek_args helper — handles optional Markdown code fences (```json ```) around the arguments payload
• Integrated into the existing _parse_tool_calls_from_text as the first-pass parser, with fallback to generic inline JSON parsing
• Supports: single tool calls, multi-tool calls, code-fenced JSON, bare JSON, surrounding text, mixed formats

Testing Plan

Unit Tests: Added 8 new tests covering:

• Single tool call with code-fenced JSON args
• Multiple tool calls in a single wrapped block
• Bare JSON args (no code fences)
• Tool call embedded in surrounding text
• Text without DeepSeek tokens (no false positives)
• Empty/whitespace-only text
• Integration test via _parse_tool_calls_from_text
• Mixed formats (DeepSeek tokens + standard inline JSON)

Regression: Full test_litellm.py: 264 passed, 0 failed

Files Changed

File	Changes
src/google/adk/models/lite_llm.py	+147 lines (2 new functions + integration)
tests/unittests/models/test_litellm.py	+124 lines (8 new test functions)

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