fix(run-templates): send plural array IDs to match API schema

[lorenjphillips]

Summary

• v1 API requires agent_ids / persona_ids / test_set_ids as arrays (min length 1), but the CLI was sending the singular agent_id / persona_id / test_set_id and every run-templates create / update call returned 400 INVALID_ARGUMENT.
• Updated request models to plural Vec<String>, response model to plural Vec<String> (also brings the cached display in line with the API response shape).
• CLI flags are now repeatable or comma-delimited:

```
coval run-templates create \
--name "Voice eval" \
--agent-ids agentA,agentB \
--persona-ids p1 --persona-ids p2 \
--test-set-ids ts1
```

Test plan

• `cargo fmt`, `cargo clippy --all-targets -- -D warnings`, `cargo test` all green locally
• After release, smoke-test `coval run-templates create` against prod

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ℹ️ Review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro

Run ID: ac710065-4e34-4d19-a73b-7f3991d18c05

📥 Commits

Reviewing files that changed from the base of the PR and between 8c7dc14 and 77cecdb.

📒 Files selected for processing (2)

• src/client/models/run_template.rs
• src/commands/run_templates.rs

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch fix/run-templates-plural-ids

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