feat: file attachment support for sending emails (#1965)

* Initial plan

* Initial exploration - understanding codebase structure

Co-authored-by: rnwood <[email protected]>

* Add attachment support for sending and replying to emails

Co-authored-by: rnwood <[email protected]>

* Address code review feedback: Add file size validation, fix memory leaks, and reduce code duplication

Co-authored-by: rnwood <[email protected]>

* Final improvements: Add XML documentation and improve exception handling

Co-authored-by: rnwood <[email protected]>

* Consolidate attachment support into existing endpoints - remove separate sendWithAttachments/replyWithAttachments endpoints

Co-authored-by: rnwood <[email protected]>

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Co-authored-by: copilot-swe-agent[bot] <[email protected]>
Co-authored-by: rnwood <[email protected]>

Author: Copilot