crypto/tls: only set LocalCertificate when a certificate is presented

[sin99xx]

ConnectionState.LocalCertificate is documented to be populated only for connections which are not resumed. However, the TLS 1.0-1.2 server handshake set it in processClientHello, which runs before checkForResumption, and nothing cleared it on the abbreviated handshake path. Resumed pre-1.3 server connections therefore reported a certificate chain that was never presented to the peer (also visible to VerifyConnection and WrapSession during the handshake).

Set the field in doFullHandshake, where the Certificate message is actually written, so it is populated if and only if a chain was presented. TLS 1.3 is unaffected because pickCertificate already returns early for PSK resumption.

Also pin MaxVersion in TestLocalCertificate and TestLocalCertificateResumption: they previously set only MinVersion, so every pre-1.3 subtest silently negotiated TLS 1.3 and the broken path had no coverage. With MaxVersion pinned, the resumption test fails without this fix at TLS 1.0-1.2.

Verified: strengthened resumption test fails on unpatched master at TLS 1.0/1.1/1.2 and passes with the fix; full go test crypto/tls passes; standalone repro from the issue prints len(LocalCertificate)=0 for resumed TLS 1.2 connections.

Fixes #79967

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• go on master (change 789862)

To reproduce, try gomote repro 8679200169796804209.

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