tools: simplify default tool search text

[sayan-oai]

Why

Default tool search text currently derives identity from both ToolName and ToolSpec. For function and namespace specs, this indexes the same names more than once and also adds a flattened {namespace}{name} token that is not model-visible.

What changed

• Derive default search text entirely from ToolSpec while preserving names, descriptions, namespace metadata, and recursive schema metadata.
• Keep the default search-text builder private and remove the unused ToolName argument.
• Add coverage for the exact search text generated for a namespaced tool with nested schema metadata.

Example

For the codex_app namespace and automation_update tool (schema terms omitted):

• Before: codex_appautomation_update automation update codex_app codex_app Manage Codex automations. automation_update automation update ...
• After: codex_app Manage Codex automations. automation_update automation update ...

Testing

• just test -p codex-tools