internal/mcp: generic handling of CallToolParams

Make Tool APIs future-proof by pushing generics into CallToolParams, and
using CallToolParams throughout. This has several advantages:

- We no longer need the awkward CallToolOptions type, which is almost
  but not quite like CallToolParams.
- NewTool no longer needs to define its own handler type: it accepts a
  ToolHandler[TArgs]. This also means its signature is future-proof.

Additionally, provide a `CallTool` helper (the analog of NewTool) to
serve as a more strongly typed variant of `ClientSession.CallTool`.

+ design, protocol generation

Change-Id: I80172a25bc50734dae84a541aa4776ed9161694e
Reviewed-on: https://go-review.googlesource.com/c/tools/+/676495
Reviewed-by: Jonathan Amsterdam <jba@google.com>
Auto-Submit: Robert Findley <rfindley@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

mcp/tool_test.go

@@ -15,7 +15,7 @@ import (
 )
 
 // testToolHandler is used for type inference in TestNewTool.
-func testToolHandler[T any](context.Context, *mcp.ServerSession, T) ([]*mcp.Content, error) {
+func testToolHandler[T any](context.Context, *mcp.ServerSession, *mcp.CallToolParams[T]) (*mcp.CallToolResult, error) {
 	panic("not implemented")
 }