[#255] Clean up remaining Phase 0 acceptance items

[eanzhao]

Closes #255

Summary

This PR finishes the acceptance cleanup that remained after the original Phase 0 landing.

It adds the missing framework-level AuthContext contract next to ICredentialProvider, and it records the honest Phase 0 provider-validation result for the current repo state: Kafka is the only durable backend validated in-repo today, while EventHubs remains a documented fallback / future validation item rather than an implied pass.

What changed

• added AuthPrincipal + AuthContext under Aevatar.Foundation.Abstractions/Credentials
• added focused abstraction tests for bot, user, and delegated auth semantics
• added ADR 0010-channel-phase0-provider-validation.md documenting the current provider-validation outcome and fallback stance
• regenerated docs/README.md and updated the abstractions README to expose the new credentials contract

Why

#255 already had DeleteAsync, ICredentialProvider, and credential_ref scaffold merged on dev, but the acceptance still was not clean because:

• the framework layer still lacked a formal AuthContext type
• the provider-validation result existed only as PR scope notes, not as a durable in-repo decision record

This PR closes both gaps without pretending EventHubs has been validated when the repository still ships Kafka-only durable backend support.

Validation

• dotnet test test/Aevatar.Foundation.Abstractions.Tests/Aevatar.Foundation.Abstractions.Tests.csproj --nologo
• bash tools/docs/build-index.sh
• bash tools/docs/lint.sh
• bash tools/ci/test_stability_guards.sh
• dotnet test test/Aevatar.Foundation.Runtime.Hosting.Tests/Aevatar.Foundation.Runtime.Hosting.Tests.csproj --filter KafkaPersistentStreamProviderRedeliveryValidationTests --nologo

Notes:

• the Kafka redelivery tests are environment-gated and skipped locally without the Kafka / Garnet env vars
• restore/build emitted existing NU1507 multi-package-source warnings unrelated to this change

Docs

• added docs/decisions/0010-channel-phase0-provider-validation.md
• regenerated docs/README.md via tools/docs/build-index.sh

[codecov]

Codecov Report

❌ Patch coverage is 81.81818% with 8 lines in your changes missing coverage. Please review.
✅ Project coverage is 68.57%. Comparing base (ec09dcc) to head (d2f28b7).
⚠️ Report is 2 commits behind head on dev.

Files with missing lines	Patch %	Lines
...Foundation.Abstractions/Credentials/AuthContext.cs	81.81%	4 Missing and 4 partials ⚠️

@@           Coverage Diff           @@
##              dev     #270   +/-   ##
=======================================
  Coverage   68.57%   68.57%           
=======================================
  Files        1109     1110    +1     
  Lines       78030    78074   +44     
  Branches    10213    10221    +8     
=======================================
+ Hits        53508    53543   +35     
- Misses      20614    20619    +5     
- Partials     3908     3912    +4     

Flag	Coverage Δ
ci	68.57% <81.81%> (+<0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
...Foundation.Abstractions/Credentials/AuthContext.cs	81.81% <81.81%> (ø)

... and 2 files with indirect coverage changes

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