[Enhancement]: Standardize tool execution results across Relay

[bbednarski9]

Goal

Establish a consistent tool-execution result contract across Relay.

Tool results should contain the application-visible result together with optional opaque annotation metadata. The contract should behave consistently across in-process, worker, and native plugin surfaces.

Expected behavior

• Tool-result annotations remain separate from the application-visible result.
• Middleware can preserve, replace, or remove annotations.
• Security and observability components can process the result and annotation independently.
• Existing continuation, error, short-circuit, and pending-mark behavior remains consistent.
• Incompatible plugin and Relay versions are rejected with clear migration guidance.

Compatibility

This introduces a breaking semantic contract for tool-execution results. The change must have a documented Relay compatibility boundary, migration guidance, and coverage across all supported plugin surfaces.

Acceptance criteria

• All tool-execution paths use the standardized result contract.
• Result and annotation behavior is consistent across plugin boundaries.
• Sanitization and export behavior are tested independently.
• Compatibility validation prevents mixing legacy and standardized result semantics.
• The migration and compatibility expectations are documented.