fix(desktop): redeploy provider agents after config saves

[skyerus]

Summary

• invoke the existing idempotent provider deploy path after a provider-backed managed agent is explicitly saved
• leave local-agent behavior unchanged (runtime changes still apply on next spawn)
• reload the managed-agent summary after deploy so provider metadata and errors are reflected in the UI
• add regression coverage proving that only provider-backed records produce a redeploy target

Closes #2798.

Why

update_managed_agent currently persists the new provider config to managed-agents.json, but the external provider is never called. Desktop therefore shows the new model/config while the deployed agent continues using the previous values. Provider protocol v1 already defines repeated deploy calls as idempotent update-in-place/no-op, so no shutdown protocol or UI lifecycle workaround is needed.

Tests

• cargo test --manifest-path desktop/src-tauri/Cargo.toml --lib --no-default-features explicit_save_redeploy_target_is_provider_backed_only
• cargo test --manifest-path desktop/src-tauri/Cargo.toml --lib --no-default-features agent_models
• cargo clippy --manifest-path desktop/src-tauri/Cargo.toml --lib --no-default-features (passes with four pre-existing secret_store.rs dead-code warnings)
• cargo fmt --manifest-path desktop/src-tauri/Cargo.toml -- --check
• git diff --check