Settle the last concrete review regressions on the release branch

A few review threads still pointed at real behavior mismatches: Anthropic tool
JSON deltas were still trimming whitespace, Responses conversion still raised
low token caps to the floor, and the bare /responses GET alias returned upgrade
messaging that did not match the actual websocket entrypoint. This commit makes
those behaviors explicit and locks them with focused tests.

Constraint: Keep the release branch green while fixing only the review findings that have a clear, testable contract
Rejected: Leave these for a follow-up cleanup PR | they were low-diff and directly blocked comment resolution on the current release PR
Confidence: high
Scope-risk: narrow
Directive: Preserve raw stream delta text unless a downstream protocol explicitly requires normalization; whitespace can be semantic in incremental payloads
Tested: Focused vitest for anthropic stream bridge, responses conversion, route aliases; typecheck; git diff --check
Not-tested: Full npm test; the remaining review threads that still need broader product decisions

src/server/routes/proxy/responses.ts

@@ -34,7 +34,7 @@ export async function responsesProxyRoute(app: FastifyInstance) {
   app.get('/responses', async (_request: FastifyRequest, reply: FastifyReply) =>
     reply.code(426).send({
       error: {
-        message: 'WebSocket upgrade required for GET /responses',
+        message: 'WebSocket upgrade required for GET /v1/responses',
         type: 'invalid_request_error',
       },
     }));

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

@@ -89,7 +89,7 @@ describe('proxy route aliases', () => {
     expect(handleOpenAiResponsesSurfaceRequestMock).not.toHaveBeenCalled();
     expect(response.json()).toEqual({
       error: {
-        message: 'WebSocket upgrade required for GET /responses',
+        message: 'WebSocket upgrade required for GET /v1/responses',
         type: 'invalid_request_error',
       },
     });