Add plugin install request schema and validation

[zswang-oai]

Why

The extension needs to describe the plural tool to the model and reject malformed picker requests before any host-side work runs.

What changed

• add flat and categorized request_plugin_installs schemas
• keep a single-entry connector schema for the TUI client
• validate entry limits, duplicate tools, category shape, known IDs, and TUI compatibility
• derive Hash for DiscoverableToolType so duplicate detection can use (tool_type, tool_id) keys

Stack

This is PR 3 of 7.

• Base: Add plugin install request model #28796
• Next: Add plugin install extension executor #28817

Validation

• just test -p codex-plugin-installs-extension (15 tests)

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Reject non-install action types

When action_type is "enable", this validator still resolves entries successfully, and build_request_plugin_installs_elicitation_request later copies args.action_type into the elicitation suggest_type. The schema says to use install, but DiscoverableToolAction deserialization accepts enable and the tool is non-strict, so malformed model calls can drive an enable suggestion through request_plugin_installs instead of being rejected before host-side work runs.

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