feat: add middleware integration with namespace isolation

[sundargthb]

Description of changes:

Add comprehensive middleware support to BedrockAgentCore SDK, enabling developers to inject cross-cutting concerns (authentication, logging, timing, metrics) into the request pipeline.

WHAT'S NEW:

• ProcessingContext: Namespaced middleware data with collision prevention
• AgentContext: Unified context combining immutable request + mutable processing
• StandardNamespaces: Constants for common middleware patterns (auth, timing, etc.)
• True immutability: RequestContext uses MappingProxyType via @Property
• Optional validation: Size limits and pattern checks (disabled by default)
• Backward compatible: Existing handlers work via property delegation

IMPLEMENTATION DETAILS:

• Namespace isolation uses Dict[str, Dict[str, Any]] structure
• MappingProxyType enforced via @Property (Pydantic v2 compatible)
• ClassVar used for class constants (Pydantic v2 requirement)
• Supports both namespaced and flat middleware data patterns
• request.state.processing_data integration in _build_request_context

TESTING:

• 8 comprehensive test suites covering all features
• Namespace isolation verified
• True immutability (MappingProxyType) verified
• Backward compatibility verified
• Authentication, timing, metrics patterns tested

┌─────────────────────────────────────────────────────────────┐
│                      AgentContext                           │
│  (What handlers receive as 'context' parameter)             │
├─────────────────────────────────────────────────────────────┤
│                                                             │
│  ┌──────────────────────────────────────────────────────┐   │
│  │ RequestContext (Immutable - Client Data)             │   │
│  │ - session_id                                         │   │
│  │ - request_headers (MappingProxyType)                 │   │
│  │ - frozen=True (cannot be modified)                   │   │
│  └──────────────────────────────────────────────────────┘   │
│                                                             │
│  ┌──────────────────────────────────────────────────────┐   │
│  │ ProcessingContext (Mutable - Middleware Data)        │   │
│  │ - middleware_data: {                                 │   │
│  │     'auth': {'user_id': 'alice'},                    │   │
│  │     'timing': {'start': 123.45},                     │   │
│  │     'metrics': {'count': 1}                          │   │
│  │   }                                                  │   │
│  │ - processing_metadata: {...}                         │   │
│  └──────────────────────────────────────────────────────┘   │
│                                                             │
└─────────────────────────────────────────────────────────────┘