Remove cosmos filter from pullrequest build definition (#38226)

Remove `sdk/cosmosdb/cosmos/` exclusion from `pullrequest.yml` trigger
paths so cosmos changes go through the standard PR validation pipeline.

A dummy `.trigger-test` file is included under `sdk/cosmosdb/cosmos/` to
ensure the PR pipeline triggers for cosmos paths.

Related to Azure/azure-sdk-tools#15160

---------

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: scbedd

eng/pipelines/pullrequest.yml

@@ -10,9 +10,6 @@ pr:
     include:
     - "*"
 
-    exclude:
-    - sdk/cosmosdb/cosmos/
-
 parameters:
   - name: Service
     type: string