feat(agent_association): add configurable description input for notification channels

[agustincelentano]

What

Exposes a new optional description input in the three agent association modules, wiring it into the nullplatform_notification_channel resource:

• parameter_storage_definition_agent_association
• scope_definition_agent_association
• service_definition_agent_association

The variable is identical across the three (type = string, default = "") for consistency.

Why

The channel description was only available (hardcoded) in the parameter storage module, and absent in scope/service. This lets consumers set a meaningful, per-channel description from the module call (e.g. driven from a for_each catalog).

Backward compatibility

⚠️ The parameter storage module previously hardcoded its description ("Notification channel to handle parameter storage and retrieval"). It now reads from var.description with default = "". Consumers relying on the old text must set it explicitly after upgrading, otherwise the channel description becomes empty.

Scope and service modules had no description before, so for them this is purely additive.

Testing

Validated end-to-end against a real nullplatform account (implementation-aws binding) by pointing the four channel modules at this branch:

• tofu validate / tofu fmt ✅
• tofu apply: 9 channels updated in-place, 0 destroyed — the API accepted the description PATCH.
• Verified via API that the description persisted on the channels (scope, service and parameter storage).

READMEs (terraform-docs ## Inputs tables) updated manually to include the new input, respecting the alphabetical sort configured in .terraform-docs.yml.

🤖 Generated with Claude Code