[Server] Add PSR-17 factory auto-discovery to StreamableHttpTransport

[CodeWithKyrian]

This PR introduces PSR-17 factory auto-discovery to the StreamableHttpTransport, simplifying HTTP transport initialization by automatically discovering ResponseFactory and StreamFactory implementations.

Motivation & Context

The original implementation required developers to manually create and pass PSR-17 factory instances, adding unnecessary boilerplate to every HTTP transport usage. This was particularly cumbersome in simple use cases and examples where the choice of PSR-7 implementation wasn't critical. By implementing auto-discovery, we can provide a much cleaner API while still allowing explicit factory injection when needed.

What's Changed

• Added php-http/discovery dependency for PSR-17 factory discovery
• Made ResponseFactory and StreamFactory constructor parameters optional with fallback to auto discovery
• Updated all HTTP examples to use the simplified API with Psr17Factory (from php-http/discovery)
• Updated documentation to explain auto-discovery behavior and supported packages
• Disabled automatic plugin installation for php-http/discovery

Breaking Changes

None. Existing code continues to work unchanged, and the simplified API is opt-in.

[chr-hertel]

Nice one - thanks! 👍