fix(types): implement custom serializer for Anthropic content blocks

[golevishal]

Motivation

The Anthropic SDK v0.84.0 introduced ParsedTextBlock, which causes Pydantic to emit PydanticSerializationUnexpectedValue warnings when event.model_dump() is called during model stream processing. The previous workaround using warnings.catch_warnings() was overly broad and risked silencing other legitimate warnings.

This PR implements a native serialization fix by applying a PlainSerializer to the content field in src/strands/types/content.py and manually extracting fields in AnthropicModel to cleanly bypass Pydantic's core validation on problematic Anthropic-specific block variants.

Resolves #1865

Public API Changes

No public API changes.

The internal serialization of AnthropicModel.stream() and the Message TypedDict have been updated:

• Added Annotated[list[ContentBlock], PlainSerializer(...)] to strands.types.content.Message.
• Intercepted event.model_dump() within src/strands/models/anthropic.py using a custom _serialize_anthropic_event() helper function.

Testing Validation

• Verified that the PydanticSerializationUnexpectedValue warnings no longer appear without needing a catch_warnings block.
• Confirmed that 2375 tests passed perfectly across the full internal test suite (hatch test).
• Ran hatch test tests/strands/models/ specifically to ensure no side-effects impacted other model integrations (OpenAI, Gemini, etc.).

Files Changed:

• src/strands/types/content.py
• src/strands/models/anthropic.py