Pydantic deprecation warning: Support for class-based `config` is deprecated, use ConfigDict instead

On import of `BedrockAgentCoreApp` a Pydantic deprecation warning occurs, e.g. during `pytest` runs. Besides noise, it is confusing since it is not clear if the issue is in application or Bedrock SDK code.

**To Reproduce**

1. open a python REPL
2. import `BedrockAgentCoreApp`

```
✗ python
Python 3.13.11 (main, Dec 5 2025, 16:06:33) [Clang 17.0.0 (clang-1700.6.3.2)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
Cmd click to launch VS Code Native REPL
>>> import warnings
>>> warnings.simplefilter("always")
>>> from bedrock_agentcore import BedrockAgentCoreApp
[REDACTED]/.venv/lib/python3.13/site-packages/bedrock_agentcore/runtime/context.py:12: PydanticDeprecatedSince20: Support for class-based `config` is deprecated, use ConfigDict instead. Deprecated in Pydantic V2.0 to be removed in V3.0. See Pydantic V2 Migration Guide at https://errors.pydantic.dev/2.12/migration/
class RequestContext(BaseModel):
```

**Expected behavior**
No deprecation warning.

**Desktop (please complete the following information):**
- OS: macOS
- Browser n/a
- Version 26.3

**Additional context**
`bedrock-agentcore` Version: 1.4.4
I'm using `pydantic` Version: 2.12.5, as required by `bedrock-agentcore`, `mcp`, `pydantic-settings`, `strands-agents`

该警告是在 bedrock_agentcore/runtime/context.py:12 定义 RequestContext 时发出的；首先阅读该类及其 Pydantic 配置。使用文档中所述的 Python REPL 导入操作重现该警告，然后验证导入 BedrockAgentCoreApp 不再发出弃用警告。

由索引模型根据 Issue 内容生成。