feat(tests): enhance modelTesterSession tests and add proxy logs tests

- Updated modelTesterSession tests to serialize assistant history as output_text and removed mime_type from input_file.
- Introduced new tests for stats proxy logs routes, covering paginated log retrieval and detailed log views with parsed billing details.
- Added server-driven tests for ProxyLogs component to validate API interactions and UI rendering based on server data.

src/server/routes/proxy/upstreamEndpoint.test.ts

@@ -501,7 +501,6 @@ describe('buildUpstreamEndpointRequest', () => {
             type: 'input_file',
             file_id: 'file_local_123',
             filename: 'paper.pdf',
-            mime_type: 'application/pdf',
             file_data: 'JVBERi0xLjQK',
           },
         ],
@@ -707,7 +706,6 @@ describe('buildUpstreamEndpointRequest', () => {
             type: 'input_file',
             filename: 'notes.txt',
             file_data: 'aGVsbG8=',
-            mime_type: 'text/plain',
           },
         ],
       },