`HelloFirefox_Auto` unexpected `tls: invalid server key share`

[hyperion-cs]

So, utls (v1.8.2) with the fingerprint HelloFirefox_Auto cannot establish a TLS connection, while in the real Firefox it works fine. Moreover, with the fingerprints HelloChrome_Auto and HelloGolang in utls, as well as in the pure golang tls library, the connection is established correctly.

PoC: https://github.com/hyperion-cs/utls-402-issue
Output:

utls chrome  => <nil>
utls firefox => tls: invalid server key share
utls golang  => <nil>
go tls       => <nil>

Please note that the PoC uses a Russian service, which may not be accessible from outside Russia or from non-resident IP addresses.

[Juktong]

I believe this is a regression of the secondary key-share management issue reported in #237. HelloFirefox_Auto offers key shares for both X25519 and P-256, but uTLS retains only the private key for the first classical key share. When the server selects P-256, uTLS attempts to process the server’s P-256 key share using the X25519 private key, resulting in tls: invalid server key share.