session: add ErrSessionNotFound and standardize not-exists handling (Fixes #85)

[casey-quinn]

Summary

• add exported session.ErrSessionNotFound sentinel for missing sessions
• standardize in-memory and database providers to wrap not-found cases with the sentinel
• update provider tests to assert errors.Is(..., session.ErrSessionNotFound)

Fixes #85

Testing

• go test ./...

References

• Branch: https://github.com/agyn-sandbox/adk-go/tree/feature/issue-85-errsessionnotfound
• Commit: https://github.com/agyn-sandbox/adk-go/commit/981277c08ede726cc9489e342feaace84bceb34a

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[gemini-code-assist[bot]]

Summary of Changes

Hello @casey-quinn, I'm Gemini Code Assist[^1]! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request refines error handling for missing sessions by introducing a dedicated sentinel error, session.ErrSessionNotFound. This change standardizes how both in-memory and database session services report when a session cannot be found, allowing consuming code to reliably identify and react to this specific condition. The update ensures consistency across different storage mechanisms and improves the clarity of error messages.

Highlights

• New Sentinel Error: Introduced session.ErrSessionNotFound, an exported sentinel error for cases where a requested session does not exist. This allows for consistent error checking using errors.Is.
• Standardized Error Handling: The in-memory and database session providers now consistently wrap 'session not found' cases with session.ErrSessionNotFound in their Get, Delete, and AppendEvent methods.
• Updated Tests: Provider tests have been updated to assert errors using errors.Is(err, session.ErrSessionNotFound), ensuring the new error handling is correctly implemented and verifiable.

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[kdroste-google]

/gemini review

[kdroste-google]

@casey-quinn, could you please look at https://github.com/google/adk-go/pull/302#issuecomment-3538984144 ? Signing CLA is required. Thank you!