fix: support file input for IM message content

[hehanlin1996]

Summary

Fix feedback C5/larksuiteGH-916 for im +messages-send so --content @card.json and --content - are resolved before JSON validation and dry-run/request construction.

Changes

• Marked the content flag as accepting common file and stdin input sources.
• Updated --content help text to document @file and stdin usage.
• Added dry-run E2E coverage for interactive message content loaded from both a JSON file and stdin.

Test Plan

• Unit tests pass: go test -buildvcs=false ./shortcuts/im -run 'TestShortcutValidateBranches|TestShortcutDryRunShapes' -count=1
• Manual local verification confirms the lark xxx command works as expected: go test -buildvcs=false ./tests/cli_e2e/im -run 'TestMessagesSendDryRun_Content(File|Stdin)$' -count=1
• Formatting checked: gofmt -l shortcuts/im/im_messages_send.go tests/cli_e2e/im/messages_send_dryrun_test.go

Related Issues

• Support file/stdin content for interactive card sends larksuite/cli#916