fix(tools): support draft-07 schema definitions

[he-yufeng]

Summary

Fixes #5940.

MCP tools may publish JSON Schema draft-07 input schemas using definitions and $ref: "#/definitions/...". ADK currently dereferences only $defs, so draft-07 schemas keep dangling refs and can crash later in Schema.from_json_schema with KeyError: 'definitions'.

This change lets the schema dereferencer read both forms:

• draft-07 definitions
• newer $defs, with $defs taking precedence when both are present

It also removes either definitions block after references have been resolved.

Validation

• .\.venv\Scripts\python.exe -m pytest tests\unittests\tools\test_gemini_schema_util.py -q
• .\.venv\Scripts\python.exe -m py_compile src\google\adk\tools\_gemini_schema_util.py tests\unittests\tools\test_gemini_schema_util.py
• .\.venv\Scripts\python.exe -m pyink --check src\google\adk\tools\_gemini_schema_util.py tests\unittests\tools\test_gemini_schema_util.py
• git diff --check

Note: running the same pytest command with the base Python environment failed during collection because that environment has an older google-genai package missing types.AvatarConfig. The repo-local .venv has google-genai==2.6.0 and passed the target tests.