Add requestModifier property to HTTPClientTransport

[mattt]

Alternative to #149, as discussed in #149 (comment)

[Copilot]

Pull Request Overview

This PR adds a requestModifier property to the HTTPClientTransport class to enable custom modification of HTTP requests before they are sent. This allows for adding authentication headers, custom headers, or other request modifications without subclassing.

• Added requestModifier parameter to HTTPClientTransport initializers with a default no-op implementation
• Applied the request modifier in both the send method and SSE connection setup
• Added comprehensive test coverage for the new functionality

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
Sources/MCP/Base/Transports/HTTPClientTransport.swift	Added requestModifier property and parameter to initializers, applied modifier in request methods
Tests/MCPTests/HTTPClientTransportTests.swift	Added test for request modifier functionality and improved test setup ordering

Comments suppressed due to low confidence (1)

Tests/MCPTests/HTTPClientTransportTests.swift:535

• This #endif directive appears to be incorrectly removed. The corresponding #if condition likely still exists above this line, creating unbalanced conditional compilation directives.

            }