[Feature Request] Validate Adaptive Card Element Objects

Problem Statement

Current Adaptive Card validation that returns errors and warnings to the caller happens at parse time. This approach leaves two scenarios unaddressed:

• If the caller constructs the object in code, the object is never validated, leaving each renderer to make it's own decisions about what happens to invalid elements that get to them.

• If the caller passes possibly invalid json with the intent to fix it later in code, they may not be able to do that as we will drop elements that are not valid at parse time.

This proposal came out of a discussion of limitations of the design we are planning for Epic #6015.

Proposed solution

Create a Validate() method on card elements that validates the current state of the card. This can then be run by each renderer at runtime, and used to drop the element and return the appropriate errors and warnings. By doing this validation at render time rather than parse time, we support API users constructing their cards in code.

Alternatives or Workarounds

If a caller wants to explicitly force validation, they could run the card through serialize/deserialize.

Each renderer currently can (needs to) validate the element itself at render time to ensure it handles missing required properties properly.

Start by reviewing the existing renderer validation paths and the serialize/deserialize workaround described in the issue. Determine how validation should apply to card elements constructed in code without dropping invalid parsed elements. Done means renderers can validate the current element state and return the appropriate errors and warnings.