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`

The warning is emitted while defining RequestContext in bedrock_agentcore/runtime/context.py:12; start by reading that class and its Pydantic configuration. Reproduce it with the documented Python REPL import, then verify that importing BedrockAgentCoreApp no longer emits the deprecation warning.

Written by the indexing model from the issue text.