Support model-specific descriptions for agent message board tools

[copyberry]

Support model-specific descriptions for agent message board tools

Why

Channel tools use bundled descriptions even when the model catalog supplies tool messages. Let their descriptions follow the active model, including model changes within a turn.

What changed

• Add channel tool entries to MultiAgentToolMessages and pass them to the message board extension for each step.
• Apply catalog descriptions while retaining bundled text for missing or null values and honoring empty strings. Ignore parameter overrides with a warning and keep bundled schemas.
• Clarify bundled descriptions for channel discovery, thread reading, posting, and subscription notifications.

Testing

Add coverage for full and partial overrides in namespaced and Code Mode tools, preserving Code Mode declarations and parameter schemas. Extend the mid-turn model-change test to verify that the post description follows the active model.

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