do not clobber other @-mentions when initial context changes

[sqs]

If you manually @-mention a file in Cody chat and then change your current editor selection, your manual @-mention is clobbered. This was a bug introduced when we changed the @-mention nodes from Lexical TextNodes to DecoratorNode-based nodes. Now we need to traverse the tree and check all nodes instead of just checking the TextNodes.

Test plan

As described above.

Changelog

• Fix a chat issue where manually entered @-mentions would be clobbered if you changed your selection or active file in the editor.