Preserve configured senders across draft writes

[jr-lillard]

Summary

• expose the optional acting sender ID on draft content
• preserve an explicitly selected sender when creating, updating, and sending drafts
• add CreateReplyDraftWithSender while retaining the existing default-sender behavior for current callers
• cover create, update, send, and reply-draft sender propagation with focused tests

Testing

• mise exec -- make check
• go test -race -count=1 ./...
• GOTOOLCHAIN=go1.26.6 govulncheck ./...

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Summary by cubic

Preserves an explicitly selected sender across draft create, update, send, and reply-draft flows, instead of always falling back to the identity's default sender.

• Adds ActingSenderID to DraftContent for message drafts; zero keeps the default-sender behavior.
• Adds CreateReplyDraftWithSender for reply drafts while existing callers keep using the default sender.
• Rejects negative acting sender IDs with a usage error on draft writes.

Looks good!

^{Written for commit baa9c85. Summary will update on new commits.}

[Copilot]

Pull request overview

Adds explicit sender preservation to draft creation, updates, sending, and reply drafts.

Changes:

• Adds ActingSenderID to draft content.
• Adds CreateReplyDraftWithSender.
• Adds focused sender propagation tests.

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Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
go/pkg/hey/messages.go	Propagates explicitly selected senders through draft writes.
go/pkg/hey/entries.go	Adds sender-aware reply draft creation.
go/pkg/hey/drafts_test.go	Tests explicit sender propagation and validation.

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