feat: register MCP author tools with registerMcpTool() + ellmer::tool()

[schloerke]

Summary

Replaces the experimental options(shiny.mcp.tools = list(...)) MCP author-tool API with an exported registerMcpTool() that accepts ellmer::tool() objects — the same tool-definition style used by mcptools and James Wade's shinymcp.

options(shiny.mcp = TRUE)

registerMcpTool(
  ellmer::tool(
    function(n) { x <- rnorm(n); list(n = n, mean = mean(x), sd = stats::sd(x)) },
    name = "get_sample_stats",
    description = "Summary statistics for a normal sample of size n.",
    arguments = list(n = ellmer::type_integer("Sample size", required = TRUE))
  )
)

Before: hand-written JSON Schema + handler, configured via a process option.
After: typed ellmer::tool() objects, registered via a function.

What changed

• New export registerMcpTool(...) — accepts one or more ellmer::ToolDef objects, stored in .globals$mcpAuthorTools (keyed by name, last-write-wins). Validates eagerly: a non-ellmer::tool() argument or a name colliding with a reserved tool (the app-opening tool or the internal _shiny_* tunnel tools) is a hard error at registration time. Name mirrors registerInputHandler().
• Schema conversion — tools/list derives each tool's inputSchema from the ToolDef's typed @arguments via ellmer's provider-agnostic as_json(), the same path as mcptools::tool_as_json() (keeps us aligned for future upstreaming).
• Invocation — tools/call runs the tool with rlang::exec(tool, !!!args). Return-value handling is unchanged (character → text, list → structuredContent, promises supported, errors → isError).
• Dependency — ellmer added to Suggests (>= 0.4.0), gated by rlang::check_installed(). Nothing on the core MCP path touches ellmer unless a tool is registered.
• options(shiny.mcp.tools) is removed — no fallback. This API was experimental and unreleased (only reached mcp, not main), so no deprecation.

Scope

Author tools only. The app-opening tool (options(shiny.mcp.tool), singular) and every other shiny.mcp.* option are intentionally unchanged — a separate follow-up.

Docs & demos

• New ?registerMcpTool help; mcp-session "Additional tools" section now points at it; pkgdown + NEWS updated.
• Demo apps (mcp/demo-app, mcp/demo-app2) and mcp/ architecture/README/QUESTIONS migrated to the new API.
• Design + implementation plan live in mcp/design-register-mcp-tool.md and mcp/implementation-plan-register-mcp-tool.md.

Testing

Full MCP test suite green (author-tool tests gated with skip_if_not_installed("ellmer")); the single skip is the CRAN-only stdio subprocess test. Covers registration/validation, tools/list schema (including required array shape), tools/call execution, error propagation, and async/promise handlers.