feat: add default parameter support

[stenalpjolly]

Summary

Adds support for default parameters in MCP tools. When a tool parameter has a "default" value defined in its JSON schema, the SDK will automatically inject this default value if the caller omits the argument during tool execution.

Expectation & Implementation

Expectation:
Tools that specify default values for their parameters should have those values automatically supplied when invoked without them, ensuring smoother user experience and aligning with standard JSON schema behavior.

Implementation:

• Updated ToolDefinition.Parameter to include a defaultValue field (mapped from the "default" JSON property).
• Modified HttpMcpToolboxClient to parse and populate the defaultValue when parsing the tool definition response.
• Enhanced Tool.validateAndSanitizeArgs to inject the defaultValue into the arguments map if the argument is null (missing) and a default is defined.

Test cases

• testDefaultValueInjection: Verifies that if an argument is omitted, its default value is automatically injected into the args map before tool invocation.
• testDefaultValueNotOverwritten: Ensures that if a caller explicitly provides a value for a parameter that has a default, the explicitly provided value is retained and not overwritten.

Acceptance criteria

• The ToolDefinition.Parameter class accurately captures the default field from the tool parameter schema.
• The Tool class automatically injects default values for missing arguments before execution.
• Explicitly provided arguments are not overwritten by their default values.
• Unit tests verify both injection and non-overwrite behaviors.
• No regressions in existing tool execution behavior.

Breaking changes

• ToolDefinition.Parameter constructor/record signature has changed to include the new defaultValue parameter. Any code directly instantiating this record will need to be updated to pass null or an appropriate default value object.

[anubhav756]

/gcbrun

[anubhav756]

/gcbrun