fix(cli, md): Unshade separator between reasoning and message blocks

[JeanMertz]

When a reasoning block has a background colour configured, the blank line separating it from the following message was incorrectly rendered with the reasoning background, visually extending the shaded region one line too far.

The root cause was that inter-block separators were emitted as part of the block that preceded them, so the separator after a reasoning block always inherited its background, regardless of what came next.

The fix defers that separator: print_block now suppresses the trailing separator for reasoning blocks (via the new
suppress_trailing_separator field on TerminalOptions) and sets a reasoning_separator_pending flag instead. When the next event arrives the flag is consumed and the separator is emitted with the correct background — shaded when more reasoning follows (the gap stays inside the reasoning region), unstyled when reasoning gives way to a message, tool call, or end of stream. flush also drains the pending separator without shading, covering role-header transitions and end-of-stream cases.

Two regression tests pin the behaviour: one checks that a single reasoning block followed by a message produces no shaded separator, and another checks that multi-paragraph reasoning keeps exactly one shaded inter-paragraph separator while the gap to the message remains unstyled.