Support for non-required MCP Tool arguments

[Azquelt]

Tool arguments are required unless one of the following is true:

• Tool.required is set to false
• Tool.defaultValue is set
• the argument type is Optional

When an argument is not required, here's how we find the value to pass into the method:

• if Tool.defaultValue is not empty, then its value is converted to the argument type using a DefaultValueConverter
  • it's an error if there is no appropriate converter, or if the converter throws an exception when converting the value
  • this conversion is implemented in MCP: Allow tool methods to have default values #33380 and Support using custom types for default values in MCP ToolArgs #33517, for this issue you should just call the converter
• if the argument type is Optional, then it's value is an empty Optional
• if the argument type is primitive, then it's value is the default value for that primitive type
• otherwise, the value is null

Ideally, the default value for each tool argument should be worked out at startup, so that we can validate it, but we probably do need to create a new instance of the value by calling the DefaultValueConverter for every tool call, since we don't know whether the object is immutable.

Tasks:

• write tests for all of the above cases
• update the Javadoc for ToolArgument to describe the behaviour listed above
• implement the necessary changes