fix(imap): align greeting CAPABILITY list with explicit CAPABILITY command

[dineth-nimnaka]

📌 Description

Aligns the capabilities advertised in the server greeting with the explicit CAPABILITY command response by reusing auth.BuildCapabilities as the single source of truth.

• Closes [TASK] Greeting CAPABILITY list differs from explicit CAPABILITY command list #261

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🔍 Changes Made

• Exported buildCapabilities as BuildCapabilities in internal/server/auth/auth.go so it can be shared across packages.
• Removed the separate/duplicate greetingCapabilities function in internal/server/server.go.
• Updated plain connection and SSL/TLS connection greetings to use auth.BuildCapabilities, ensuring NAMESPACE, UNSELECT, and all future capabilities stay in sync across greeting and explicit command responses.

✅ Checklist (Email System)

• Core IMAP commands tested (LOGIN, CAPABILITY, LIST, SELECT, FETCH, LOGOUT).
• Authentication is tested.
• Docker build & run validated.
• Configuration loading verified for default and custom paths.
• Persistent storage with Docker volume verified.
• Error handling and logging verified
• Documentation updated (README, config samples).

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🧪 Testing Instructions

To test the server, use the instructions in the README in the test directory.

1. Connect to Raven via plain socket (port 143) and observe the initial * OK [CAPABILITY ...] greeting.
2. Send CAPABILITY command and compare the capability list; verify they are aligned (including NAMESPACE and UNSELECT).
3. Connect via TLS/SSL and repeat to verify both TLS states match.

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📷 Screenshots / Logs (if applicable)

N/A

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⚠️ Notes for Reviewers

Refactored greeting to use auth.BuildCapabilities directly to eliminate code duplication and avoid capability drift in the future.

[CLAassistant]

All committers have signed the CLA.

[greptile-apps]

Confidence Score: 5/5

[Medium impact] The PR appears safe to merge with no outstanding findings.

Diagram

%%{init: {'theme': 'neutral'}}%%
flowchart LR
    Plain["Plain greeting"] --> Builder["auth.BuildCapabilities"]
    TLS["TLS greeting"] --> Builder
    Command["CAPABILITY command"] --> Builder
    Builder --> List["Shared capability list"]

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_{Reviews (4) · Last reviewed commit: "fix(lint): remove unused oauthSASLReady ..." · Reviewed by Greptile}

[maneesha-xyz]

Hi @dineth-nimnaka we have made some changes to our CLA. We need contributors to sign it again. Please let me know if there is any issue.

[dineth-nimnaka]

Signed the updated CLA! @maneesha-xyz