fix: allow renaming an agent to its current name

[Fail-Safe]

Summary

• Fixes AgentRegistry::update_name incorrectly rejecting a rename when the new name is the same as the current name

Problem

update_name checks name_index.contains_key(&new_name) before proceeding. Since an agent's current name is always in the index, renaming to the same name always hit the AgentAlreadyExists error path — even though no conflict exists.

This surfaces in the dashboard whenever a user edits an agent's name and saves without changing it (e.g. after editing another field in the same form), producing an unexpected error.

Fix

Check whether the name is owned by a different agent before erroring. If the same agent already owns the name, return Ok(()) immediately.

// Before
if self.name_index.contains_key(&new_name) {
    return Err(OpenFangError::AgentAlreadyExists(new_name));
}

// After
if let Some(existing_id) = self.name_index.get(&new_name).as_deref().copied() {
    if existing_id != id {
        return Err(OpenFangError::AgentAlreadyExists(new_name));
    }
    return Ok(());  // same agent — no-op
}

Test plan

• Rename an agent to its current name — should succeed silently
• Rename an agent to a name already used by a different agent — should still return AgentAlreadyExists
• Rename an agent to a genuinely new name — should succeed and update the index

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