Use borrowed zero-allocation check for search capability in PluginManager::search_filtered

[multiplex55]

Motivation

• Avoid allocating a String just to check whether a plugin's enabled capabilities include "search", and ensure plugins without that capability are skipped during search routing.

Description

• Replace caps.contains(&"search".to_string()) with a zero-allocation borrowed comparison caps.iter().any(|c| c == "search") in PluginManager::search_filtered.
• Add unit test search_capability_gate_skips_plugin_when_disabled in tests/plugin_routing.rs to verify a plugin is skipped when its enabled capabilities do not include "search".
• Did not migrate capability storage to a HashSet in this change; Vec<String> callsites remain unchanged for minimal scope.

Testing

• Ran cargo fmt --all, which succeeded.
• Attempted cargo test --test plugin_routing and cargo test --test plugin_routing --no-default-features, but both failed in this environment due to a missing system alsa development library required by alsa-sys, so the new test could not be executed here.

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Codex Task