.NET: allow a custom JsonSerializerOptions (source-generated) for agent-hooks wire projection

Feature request

Follow-up from review of #7564 (https://github.com/microsoft/agent-framework/pull/7564#discussion_r3803635610).

The agent-hooks enforcement package projects framework values (messages, rich content, tool arguments and results) into AGENT-HOOKS-0.1 wire JSON using reflection-based System.Text.Json via AIJsonUtilities.DefaultOptions (see Wire.JsonOptions in dotnet/src/Microsoft.Agents.AI.AgentHooks/Wire.cs). Because tool arguments/results are arbitrary user types, the project currently sets IsAotCompatible=false.

Proposal: allow a developer to supply a custom JsonSerializerOptions — built with source-generated JSON serialization — for the wire projection (e.g. an AgentHooksOptions.JsonSerializerOptions property defaulting to AIJsonUtilities.DefaultOptions). That gives source-gen/AOT-oriented applications a path to full trimming/AOT compatibility for this package and mirrors how the framework itself threads serializer options through its own surfaces.

Considerations:

• The projection must stay faithful and fail-closed: a value the supplied options cannot serialize should keep the current repr-fallback (or fail closed at the guarded seams), never crash with a trail gap.
• The write-back direction (Wire.WireToContents) deserializes AIContent polymorphically and must use the same options.
• The ResponsibleAI.AgentHooks SDK itself operates on JsonNode and is unaffected.