fix: guard TeamServiceBase against null current user

[poxet]

Summary

Loading a <TeamComponent /> page before authentication (or after session expiry) crashed the Blazor circuit with NullReferenceException. TeamServiceBase.GetTeamsAsync<TMember>() took the result of GetCurrentUserAsync() and passed it straight to the MongoDB GetTeamsAsync(IUser) override, which dereferenced user.Key. The same unguarded GetCurrentUserAsync pattern existed at 7 call sites in TeamServiceBase.

Each call site now handles a null current user according to its semantic:

Method	Behaviour on null user
GetTeamsAsync() (non-generic)	yield break (empty stream)
GetTeamsAsync<TMember>()	yield break
CreateTeamAsync(string name)	throw UnauthorizedAccessException("Authentication required.")
RemoveMemberAsync	throw
SetMemberLastSeenAsync	early-return (touch op, benign no-op)
TransferOwnershipAsync<TMember>	throw
AssureAccessLevel<TMember> (private gate)	throw

A new private RequireCurrentUserAsync() helper centralises the throw; the existing GetCurrentUserAsync() is kept for read paths that handle null themselves.

A defensive null guard at the data layer was also added: TeamServiceRepositoryBase.GetTeamsAsync(IUser user) returns AsyncEnumerable.Empty<ITeam>() for null input. Belt-and-suspenders so the same bug class can't be reintroduced from a future caller that forgets the upstream guard.

Consumer impact

• No public API change. Both helpers are private to TeamServiceBase.
• UnauthorizedAccessException is System.UnauthorizedAccessException (not a custom Tharga type) so it integrates cleanly with ASP.NET pipelines that map it to 401.
• Pages hosting <TeamComponent /> rendered to an unauthenticated principal now show the empty state ("You are not member of a team.") instead of crashing the circuit. Authenticated flow is unchanged.

Branch contents

This branch carries three commits to master:

• fa36d13 feat: invited-member-edit-fix complete — close-out for #62.
• ed7346f fix: guard TeamServiceBase against null current user — this fix.
• 4e9aeb8 feat: unauthenticated-team-service-guards complete — finalize commit removing the feature's working plan/ so master never carries it.

Test plan

• dotnet build -c Release -- 0 warnings, 0 errors.
• dotnet test -c Release -- 280 / 280 passing (was 274, +6 new in UnauthenticatedTeamServiceTests):
  • GetTeamsAsync_Unauthenticated_ReturnsEmpty
  • GetTeamsAsync_Generic_Unauthenticated_ReturnsEmpty
  • CreateTeamAsync_Unauthenticated_Throws
  • RemoveMemberAsync_Unauthenticated_Throws (targets a non-owner row -- the owner-protection check fires first for the owner row)
  • SetMemberLastSeenAsync_Unauthenticated_DoesNotThrow
  • TransferOwnershipAsync_Unauthenticated_Throws
• Manual: load the sample app team page in an incognito session; confirm no NRE in the host console and the empty state renders. Sign in and confirm authenticated flow is unchanged.

[codecov]

Codecov Report

❌ Patch coverage is 90.90909% with 1 line in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
Tharga.Team/TeamServiceBase.cs	90.90%	0 Missing and 1 partial ⚠️

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