dio Files

## What's new

• Expose supportedProtocols on ConnectionManager (defaults to ['h2'], fully backwards-compatible). Setting it to ['h2', 'http/1.1'] fixes fallbackAdapter silently never firing against RFC 7301-compliant servers that abort the TLS handshake with no_application_protocol when only h2 is advertised.
• Fix socket leak: when DioH2NotSupportedException is thrown after a successful TLS handshake (server selected http/1.1 instead of h2), the SecureSocket is now explicitly destroyed before routing to fallbackAdapter, rather than being abandoned.
• Deprecate the misspelled ConnectionManager.handshakeTimout parameter in favor of the correctly spelled handshakeTimeout; the deprecated alias keeps working and is scheduled for removal in 3.0.0.