pam/nativemodel: Ensure we're in the expected stage before initiating it

[3v1n0]

Fixed handling of focus for authentication stage when not using the CLI interface, and fix the race we had for some time, making impossible to go back to auth-selection if going back from the challenge stage was fast enough (VHS quick typing speed was a good friend here :)).

From main commit:

In some cases (when the "goBack{}" request via the 'r' cancel key is
performed very quickly on the UI) we might end up doing a
nativeAuthSelection{} request when we are still technically performing
the stage change from the challenge stage to auth-mode selection.

This was particularly a problem in the "Wait" cases, given that these
events could have been arriving without a proper order, because the
cancellation of the previous event could lead to a delayed stage-change
request that we didn't handle, but it could happen also in non-wait
cases, for example:

  authd-pam-exec-DEBUG: 23:46:32.560: authenticate: called method
    Prompt((1, 'Gimme your password:\n> '))
  Native model update: tea.sequenceMsg{(tea.Cmd)(0xca47c0),
    (tea.Cmd)(nil)}
  Native model update: adapter.nativeGoBack{}
  adapter.isAuthenticatedCancelled{msg:""}
  Native model update: adapter.isAuthenticatedCancelled{msg:""}
  Native model update: adapter.nativeStageChangeRequest{Stage:2}
  Native model update: tea.sequenceMsg{(tea.Cmd)(0xca47c0),
    (tea.Cmd)(0xca47c0)}
  Native model update: adapter.nativeChangeStage{Stage:3}
  *adapter.ChangeStage{Stage:2}
  *adapter.authenticationModel: Reset"
  *adapter.authModeSelectionModel: Focus"
  Native model update: adapter.nativeAuthSelection{}
  Native model update: tea.sequenceMsg{(tea.Cmd)(0xca47c0),
        (tea.Cmd)(0xc96b00), (tea.Cmd)(0xca47c0)}
  Native model update: adapter.nativeChangeStage{Stage:2}
  adapter.authModeSelectionModel: adapter.authModeSelectionFocused{}
  Native model update: adapter.authModeSelectionFocused{}

or:

  authenticate: called method Prompt((2, 'Choose action:\n> '))
  Native model update: adapter.nativeChangeStage{Stage:3}
  Native model update: tea.sequenceMsg{(tea.Cmd)(0x1496660),
    (tea.Cmd)(nil)}
  Native model update: adapter.nativeGoBack{}
  adapter.isAuthenticatedCancelled{msg:""}
  Native model update: adapter.isAuthenticatedCancelled{msg:""}
  Native model update: adapter.nativeStageChangeRequest{Stage:2}
  Native model update: adapter.nativeChangeStage{Stage:3}
  Native model update: tea.sequenceMsg{(tea.Cmd)(0x1496660),
    (tea.Cmd)(0x1496660)}
  adapter.ChangeStage{Stage:2}
  *adapter.authenticationModel: Reset
  *adapter.authModeSelectionModel: Focus
  Native model update: adapter.nativeAuthSelection{}
  Native model update: tea.sequenceMsg{(tea.Cmd)(0x1496660),
    (tea.Cmd)(0x1483980), (tea.Cmd)(0x1496660)}
  Native model update: adapter.nativeChangeStage{Stage:2}
  adapter.authModeSelectionModel: adapter.authModeSelectionFocused{}
  Native model update: adapter.authModeSelectionFocused{}

As visible, the nativeStageChangeRequest (which triggered the selection
via nativeAuthSelection{}) was arriving *before* that the request of
changing the internal stage value (via nativeChangeStage{}) was actually
delivered. And this prevented nativeAuthSelection{} to anything (or any
other stage-change request in general).

So, basically we need to ensure that the request is happening only after
the internal stage is changed, and this is now fixed by relying on the
main StageChange event request that eventually will update the internal
state, after we're triggering again nativeStageChangeRequest{}.

Closes: #719

UDENG-6191

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 48.78049% with 21 lines in your changes missing coverage. Please review.

Project coverage is 79.05%. Comparing base (36511cd) to head (dadebf9).
Report is 449 commits behind head on main.

Files with missing lines	Patch %	Lines
pam/internal/adapter/nativemodel.go	37.50%	9 Missing and 6 partials ⚠️
pam/internal/adapter/focustracker.go	60.00%	6 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #818      +/-   ##
==========================================
- Coverage   83.43%   79.05%   -4.39%     
==========================================
  Files          83      103      +20     
  Lines        8689    10406    +1717     
  Branches       74       74              
==========================================
+ Hits         7250     8226     +976     
- Misses       1111     1715     +604     
- Partials      328      465     +137     

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