Harden runtime error paths and standardize API config reload semantics

[vsumner]

Summary

This PR started as failing-test + panic hardening and now also standardizes config mutation/reload semantics across API endpoints, with new targeted coverage.

What Changed

Test and runtime hardening

• Fixed failing tests/doctests and metadata-only search test setup.
• Hardened runtime paths by replacing panic-prone expect/unwrap usage with guarded error handling in channel/worker/config/messaging paths.
• Hardened channel prompt handling and streaming lock safety.
• Removed remaining prompt-construction panic paths in channel and worker.
• Applied rustfmt cleanup required by CI.

API config mutation standardization

• Added shared helpers in src/api/config.rs:
  • write_validated_config (validate TOML -> write -> reload parsed Config)
  • reload_all_runtime_configs (reload all live RuntimeConfig instances)
  • sync_bindings_and_permissions (sync bindings ArcSwap + Discord/Slack permissions)
• Refactored endpoints to use the shared flow:
  • update_agent_config
  • update_provider, delete_provider
  • toggle_platform, disconnect_platform
  • create_binding, update_binding, delete_binding
• Removed ad-hoc best-effort reload branches in favor of explicit validated behavior.

New API coverage for standardized behavior

• Added targeted api::bindings tests to verify:
  • invalid config mutation returns 400 BAD_REQUEST
  • provider update writes config and reloads live runtime routing
  • binding create writes config and syncs in-memory bindings + Discord/Slack permissions

Validation

• cargo fmt --all -- --check
• cargo test api::bindings::tests:: -- --nocapture
• cargo test