Follow-Up: Schema-Based Serialization Improvements

Is your feature request related to a problem? Please describe.
There are a few open follow-up tasks related to the schema-based serialization utilities:

• The functions _serialize_value_with_schema and _deserialize_value_with_schema are currently private. The plan is to make them public once they are verified to work reliably in scenarios like:
  -- Breakpoint input serialization
  -- Agent State serialization

• The current implementation does not support lists with mixed types (e.g., [1, "a", {"x": 2}]). We should either:
  Decide explicitly that mixed-type lists are out of scope, and document that limitation
  Or, extend the implementation to handle them correctly

Describe the solution you'd like

• Promote _serialize_with_schema and _deserialize_with_schema to public utility functions after validation in real use cases
• Resolve the handling of mixed-type lists: either by implementing support or documenting the constraint

Reference discussion: https://github.com/deepset-ai/haystack/pull/9526#discussion_r2153806997

Start by reading the reference discussion on pull request 9526 and tracing _serialize_value_with_schema and _deserialize_value_with_schema in their current uses. Validate them against breakpoint input serialization and Agent State serialization, then decide whether mixed-type lists are supported or explicitly documented as out of scope; done means the public API and limitation are settled.