agent_ui: Insert thread summary as proper mention URI

[pkondzior]

This ensures the thread summary is treated as a tracked mention with accessible context.

Changes:

• Fixed MessageEditor::insert_thread_summary() to use proper mention URI format
• Added test coverage to verify the fix

Release Notes:

• Fixed an issue where "New From Summary" was not properly inserting thread summaries as contextual mentions when creating new threads. Thread summaries are now inserted as proper mention URIs.

[bennetbo]

It's not clear to me how it would solve the issue the user reported. The current format should be the same as after this PR? Can you give some more details?
On my end "New from summary" is working as expected

[pkondzior]

@bennetbo I dont think this should be closed without reconsidering that every other place in the code inserts mentions as link, but only summary is being inserted as title, this PR fixes it.

[bennetbo]

Looking at it again your change makes sense to get in. However, I don't see how it would fix the bug that was reported in #38292, since this PR does not change anything of how we include a summary. Correct me if I'm missing anything.
Happy to merge this if the conflicts are fixed and we drop the Closes #38292.

[pkondzior]

@bennetbo thanks, I've redacted description, I will also handle merge conflicts. I am also unable to reproduce original issue regarding #38292 but I am definitely able to reproduce issue with copy-pasting mention that this PR fixes following video shows whats impossible right now on upstream branch

Screen.Recording.2025-10-30.at.02.15.32.mov

[pkondzior]

@bennetbo I've addressed the failure on windows/linux which seems not include initiation LSP fake server, I'm adding init harness only to my test but if you recommend to move this into init_test I can do that

cx.update(LanguageModelRegistry::test);

[bennetbo]

Thank you!