fix: restore the credential-rejected branch and admit SRC absence by key

Review follow-ups that are logic rather than documentation:

- rpc.schema.ts had lost the credential-rejected branch while api-proxy.ts
  still returns that code, so a legitimate business error failed the
  client's response parse. Restore the branch and assert it.
- rpc-schemas.spec.ts had lost the workspace list, archiveSession and
  insertSessionBefore cases along with the command schemas; those routes
  still ship, so restore their coverage.
- An omitted SRC field is now recognized by an absent key instead of an
  undefined value, which makes the allowance assertExactArguments already
  granted reachable; an explicitly undefined field stays invalid input. A
  weak descriptor's undefined result rides the wire as an absent value,
  matching the envelope removal.
- The chooser unmounts an already-created backend when the surface entry
  fails to load, and no longer reverses the captured id array in place.

packages/api/gateway/tests/gateway.spec.ts

@@ -788,6 +788,19 @@ describe('TypertGatewayService', () => {
     }), 'input-invalid')
   })
 
+  it('admits an omitted SRC field and hands the Host method undefined', async () => {
+    const { ctx, service } = await setup()
+    // A weak descriptor reads parameter names from the JavaScript signature and
+    // cannot see which are optional, so an absent field is admitted; the case
+    // above keeps an explicitly undefined field rejected.
+    await expect(ctx.typertGateway.invoke({
+      namespace: 'goals',
+      method: 'passthrough',
+      args: {},
+    })).resolves.toBeUndefined()
+    expect(service.calls).toContain('passthrough')
+  })
+
   it('rejects cyclic SRC input and non-JSON SRC results', async () => {
     const { ctx, service } = await setup()
     const cyclic: { self?: unknown } = {}