add a method to Messages to set a cache breakpoint

[elimoss]

No description provided.

[Copilot]

Pull request overview

Adds support for marking a Message as a “cache breakpoint” so that serialized content includes cache_control metadata for prompt caching (e.g., Anthropic).

Changes:

• Introduces a cache_breakpoint flag on Message, excluded from dumps but used during serialization to inject cache_control.
• Adds Message.set_cache_breakpoint(enabled: bool = True) for fluent enabling/disabling.
• Adds unit tests for default behavior and serialization outcomes, plus a new live integration-style test.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 3 comments.

File	Description
src/aviary/message.py	Adds cache_breakpoint field, serialization behavior to inject cache_control, and a setter method.
tests/test_messages.py	Adds test coverage for cache_breakpoint serialization and behavior, including a new live network test.

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[jamesbraza]

Nice work! Thanks for doing this