Make tool executor specs mandatory

[jif-oai]

Why

ToolExecutor is the runtime contract that keeps a callable tool and its model-visible spec together. Leaving spec() optional lets a registered runtime silently omit that half of the contract, and it also overloads a missing spec as an exposure decision for tools that should stay dispatchable without being shown to the model.

What

• Make ToolExecutor::spec() required and update core, extension, and test tool executors to return a concrete ToolSpec.
• Add ToolExposure::Hidden for dispatch-only tools. The legacy shell_command runtime in unified-exec sessions now uses that explicit exposure instead of hiding itself by omitting a spec.
• Build MCP tool specs when McpHandler is constructed so invalid MCP specs are skipped before the handler is registered.
• Keep tool planning aligned with the new contract for direct, deferred, hidden, code-mode, dynamic, and namespaced tool paths.

Testing

• Added tool-plan coverage that invalid MCP tool specs are not registered.
• Updated shell-family coverage for the hidden legacy shell_command runtime and the affected tool executor test fixtures.

[jif-oai]

@codex review

[jif-oai]

Windows failures are due to GH workers flake