Clarify conversation-context counts after session delta filtering #5421

Problem

Session-aware prompt delivery can filter messages that the live ACP session already received. The rendered heading still uses the post-filter message count with the pre-filter total and truncation state, for example:

[Thread Context (1 of 5 messages)]

That does not distinguish messages withheld because they were delivered earlier in the session from messages omitted by the fetch window.

Desired outcome

Make the conversation-context heading distinguish:

• messages included in this prompt;
• messages withheld because the current session already received them; and
• messages omitted because the fetch was truncated.

Follow-up from #5342 session-context delivery work; intentionally kept out of that bounded PR because the rendering semantics need a product/copy decision.