fix(acp): reject ineligible authentication methods

[YoungSx]

Summary

• allowlist ACP auth-method discriminators so only agent methods reach authenticate
• enforce the same rule at the protocol client's outgoing wire boundary
• add localized unsupported-method guidance and protocol/selection/transport regression coverage

Verification

• dotnet build src/SalmonEgg.Presentation.Core/SalmonEgg.Presentation.Core.csproj -c Release --nologo
• dotnet test --project tests/SalmonEgg.Acp.Tests/SalmonEgg.Acp.Tests.csproj -c Release --timeout 10m --output Normal (346/346)
• dotnet test --project tests/SalmonEgg.Application.Tests/SalmonEgg.Application.Tests.csproj -c Release --timeout 8m --output Normal (120/120)
• Focused Presentation.Core auth tests (12/12) and localization tests (209/209)

The full Presentation.Core host has pre-existing load-sensitive instability outside this change: a StartViewModel timeout and an unhandled ChatViewModel cancellation can terminate the host (exit 134).

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Project	Deployment	Actions	Updated
salmonegg	Ready	Preview	Sep 1, 2026 5:31pm UTC