github-authentication - simplify avatar persistence

[Dmitriy Vasyura (dmitrivMS)]

Summary

Follow-up to #308654, which introduced authSessionAccountIcon and account avatar support.

This PR applies Tyler's review cleanup to the GitHub Authentication session-persistence implementation:

• Derives the stored icon component type from vscode.Uri.from instead of maintaining a duplicate URI-components interface.
• Relies on native vscode.Uri JSON serialization instead of manually rebuilding stored URI components.
• Stops making /user requests solely to backfill icons for sessions that already contain account information.
• Removes the no-avatar null sentinel, WeakSet cache, manual pruning, serialization helpers, and their helper-only tests.

New sessions and legacy accountless sessions still receive and persist avatars during their already-required /user lookup. Existing accountful sessions created before avatar support remain valid and iconless until naturally recreated or refreshed.

Validation

• GitHub Authentication extension compilation passed.
• ESLint passed.
• GitHub Authentication extension suite: 26 passing.
• git diff --check passed.

[Copilot]

Pull request overview

Simplifies avatar persistence in the GitHub Authentication provider while preserving avatar lookup for new and legacy accountless sessions.

Changes:

• Uses native URI serialization and inferred URI component types.
• Removes redundant avatar lookup, caching, and serialization helpers.
• Removes helper-specific tests made obsolete by the simplification.

Show a summary per file

File	Description
extensions/github-authentication/src/github.ts	Simplifies session and avatar persistence.
extensions/github-authentication/src/test/github.test.ts	Removes obsolete helper tests.

Review details

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• Files reviewed: 2/2 changed files
• Comments generated: 0
• Review effort level: Balanced