fix: Keep new session content visible

[xingkaixin]

Live session filters stored the categories present at first render as the complete selection. A first assistant reply or newly used tool was therefore hidden even when the reader had not applied a filter.

Store only explicit exclusions and derive visible categories from the current session contents. Existing choices survive live updates, while new categories remain visible. Bulk actions retain their current-category scope, and changing sessions or resetting clears the exclusions. Tool search and expansion do not trigger message filtering again.

Validation:

• Reproduced a live reply disappearing before the change and verified both messages remain visible afterward.
• Added coverage for new replies/content/tools, retained exclusions after categories disappear and return, and bulk-action behavior.
• Passed local lint, formatting, typecheck, build, and tests.