codex-tools: extract discoverable tool models

[bolinfest]

Why

#16193 moved the pure tool_search and tool_suggest spec builders into codex-tools, but codex-core still owned the shared discoverable-tool model that those builders and the tool_suggest runtime both depend on. This change continues the migration by moving that reusable model boundary out of codex-core as well, so the discovery/suggestion stack uses one shared set of types and core/src/tools no longer needs its own discoverable.rs module.

What changed

• Moved DiscoverableTool, DiscoverablePluginInfo, and filter_tool_suggest_discoverable_tools_for_client() into codex-rs/tools/src/tool_discovery.rs alongside the extracted discovery/suggestion spec builders.
• Added codex-app-server-protocol as a codex-tools dependency so the shared discoverable-tool model can own the connector-side AppInfo variant directly.
• Updated core/src/tools/handlers/tool_suggest.rs, core/src/tools/spec.rs, core/src/tools/router.rs, core/src/connectors.rs, and core/src/codex.rs to consume the shared codex-tools model instead of the old core-local declarations.
• Changed core/src/plugins/discoverable.rs to return DiscoverablePluginInfo directly, moved the pure client-filter coverage into tool_discovery_tests.rs, and deleted the old core/src/tools/discoverable.rs module.
• Updated codex-rs/tools/README.md so the crate boundary documents that codex-tools now owns the discoverable-tool models in addition to the discovery/suggestion spec builders.

Test plan

• cargo test -p codex-tools
• CARGO_TARGET_DIR=/tmp/codex-core-discoverable-model cargo test -p codex-core --lib tools::handlers::tool_suggest::
• CARGO_TARGET_DIR=/tmp/codex-core-discoverable-model cargo test -p codex-core --lib tools::spec::
• CARGO_TARGET_DIR=/tmp/codex-core-discoverable-model cargo test -p codex-core --lib plugins::discoverable::
• just bazel-lock-check
• just argument-comment-lint

References

• codex-tools: extract discovery tool specs #16193
• codex-tools: extract utility tool specs #16154
• codex-tools: extract shared tool schema parsing #15923
• codex-tools: extract MCP schema adapters #15928
• codex-tools: extract dynamic tool adapters #15944
• codex-tools: introduce named tool definitions #15953
• codex-tools: extract responses API tool models #16031
• codex-tools: extract tool spec models #16047
• codex-tools: extract configured tool specs #16129
• codex-tools: extract code mode tool spec adapters #16132
• codex-tools: extract local host tool specs #16138
• codex-tools: extract collaboration tool specs #16141