fix(sio-client): export acknowledgement decorator for declaration emit

[ascebiswajit]

Current behavior

Exporting a mapped type of a Socket’s public properties fails during TypeScript declaration generation with TS2883 because DecorateAcknowledgements is not exported from the package entry point.

Related to the remaining error reported in #5307.

New behavior

• Export DecorateAcknowledgements from socket.io-client.
• Add a mapped-type regression example.
• Enable declaration generation in the existing public-exports example.
• Run the declaration check in client CI.

Runtime behavior is unchanged.

Validation

• Regression check fails before the fix and passes afterward.
• Compilation and formatting pass.
• All 112 Node tests pass locally and in the fork’s CI.
• Browser tests could not start: the fork lacks Sauce Labs credentials, and EdgeDriver version resolution also failed.