ssh: reject incomplete gssapi-with-mic configurations

[povcfe]

Make the runtime gssapi-with-mic guard match the existing
configuration and method advertisement checks.

An incomplete GSSAPIWithMICConfig can be treated as unavailable when
building the advertised auth method list, while still remaining
reachable from the runtime auth dispatcher. Treat incomplete
configurations as not configured.

This change introduces a single internal completeness check for
GSSAPIWithMICConfig and uses it for the startup authentication
validation, the runtime gssapi-with-mic dispatch guard, and the
advertised authentication method list.

The change also adds a regression test. The test configures a server
with a normal PasswordCallback, a GSSAPIWithMICConfig with Server set,
and AllowLogin intentionally unset. It then uses a custom client auth
method that explicitly sends a USERAUTH_REQUEST with Method set to
gssapi-with-mic even though the server does not advertise that method,
and verifies that authentication fails cleanly with
"ssh: gssapi-with-mic auth not configured".

No golang/go issue reference is available yet.

[gopherbot]

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[gopherbot]

Message from Gopher Robot:

Patch Set 1:

(1 comment)

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Patch Set 1:

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Message from Nicola Murino:

Patch Set 2: Commit-Queue+1

(2 comments)

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Message from Go LUCI:

Patch Set 2:

Dry run: CV is trying the patch.

Bot data: {"action":"start","triggered_at":"2026-05-03T18:44:17Z","revision":"c8130c1bec774d04b995e0018b97b5785fd0d6d2"}

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Message from Nicola Murino:

Patch Set 2: -Commit-Queue

(Performed by <GERRIT_ACCOUNT_60063> on behalf of <GERRIT_ACCOUNT_35201>)

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Patch Set 2:

This CL has passed the run

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Patch Set 2: LUCI-TryBot-Result+1

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This PR (HEAD: 0267bda) has been imported to Gerrit for code review.

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Message from Filippo Valsorda:

Patch Set 3: Code-Review+2

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Message from povcfe:

Patch Set 3: Code-Review+1

(3 comments)

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