fix: clarify IM message dry-run limits

[hehanlin1996]

Summary

Clarifies two IM shortcut UX gaps from the P1 IM/Agent collaboration evidence: im +messages-send --dry-run now explicitly warns that chat membership is not verified, and IM media flags now document the cwd-relative local path restriction.

Changes

• Add a dry-run _notice.im_chat_membership warning for im +messages-send --chat-id --dry-run, while leaving direct --user-id sends unchanged.
• Preserve media-upload dry-run descriptions and append the chat-membership caveat for pretty/description output.
• Update +messages-send and +messages-reply media flag help to say local file paths must be cwd-relative and absolute paths are rejected.
• Add unit coverage and IM dry-run E2E coverage for the new notice/help behavior.

Test Plan

• TDD red: new IM dry-run/help unit tests failed before implementation.
• go test ./shortcuts/im -count=1
• go test -race -gcflags="all=-N -l" ./shortcuts/im -count=1
• go build -buildvcs=false -o lark-cli . && LARK_CLI_BIN=$PWD/lark-cli go test ./tests/cli_e2e/im -run 'TestIM_MessagesSend(DryRunWarnsChatMembershipNotVerified|HelpDocumentsMediaPathRestriction)' -count=1 -v
• gofmt -l shortcuts/im tests/cli_e2e/im/messages_send_dryrun_test.go produced no output
• go vet ./shortcuts/im ./tests/cli_e2e/im
• go mod tidy made no changes
• make unit-test attempted; unrelated existing failure in shortcuts/base (TestBaseRecordExecuteReadCreateDelete/upload_attachment_uses_multipart_for_large_file, MIME detected as application/x-sega-cd-rom for large-report.bin). IM package passed in the same run.

Related Issues

• Refs Clarify that im messages-send --dry-run does not verify chat membership larksuite/cli#915
• Refs --image / --file flags reject absolute paths but --help doesn't document this larksuite/cli#872