GmailController now resolves the "gmail" provider from IEmailProviderRegistry and
uses the provider-neutral seam for its read paths — message search (SearchAsync)
and thread listing (ListThreadMessagesAsync) across ImportThread, RelinkThread,
CreateSuggestedJob, RefreshLinkedThreads and the messages endpoint. OAuth
(connect/callback), connection status and Gmail-specific candidate ranking stay
on IGmailOAuthService until they are generalised.
An optional constructor param keeps direct construction (tests) working via a
fallback single-Gmail registry, so the mocked Gmail service is exercised through
GmailProvider. Behaviour is preserved (neutral DTOs mirror the Gmail shapes).
This makes the seam a real consumer and sets up MicrosoftGraphProvider /
ImapProvider / a manual free-text provider to slot in next.
Build clean; backend suite 135/135 green.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
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