Add fallback for whoami when People API is disabled

[cct08311github]

Summary

• fall back to token-based profile lookup when people me/whoami hits 403 accessNotConfigured
• keep the normal People API path when it is enabled
• add a regression test covering the fallback path

Testing

• /opt/homebrew/bin/go test ./internal/cmd -run PeopleMe
• /opt/homebrew/bin/go test ./cmd/gog

Fixes #461

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Preserve the active auth mode in the People API fallback

If people/me hits accessNotConfigured while the caller is using a supported non-keyring auth path (--access-token, ADC, or service-account impersonation), this helper throws those credentials away and starts a fresh keyring lookup from account. newPeopleContactsService explicitly prefers those context credentials in tokenSourceForAvailableAccountAuth, so gog whoami --access-token ... --account bob@example.com can now report Bob's stored OAuth profile instead of the identity behind the supplied token, or fail with unrelated resolve client/get token errors.

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