fix(mitm): align hookBufferUpdate caller type with sourceModel runtime (R4 #4)

The local type annotation for `recordRequestStart` in `loadAgentBridgeHook`
omitted `sourceModel`, but the call site at line 217-222 passes
`sourceModel: this.extractSourceModel(body)` — which works at runtime
(JavaScript ignores extra properties) but a future strict-mode caller relying
on the narrower local type would silently drop the field.

Add `sourceModel?: string | null` to the local recordRequestStart type to
match the actual `agentBridgeHook.recordRequestStart` shape.

src/mitm/handlers/base.ts

@@ -53,6 +53,7 @@ async function loadAgentBridgeHook(): Promise<{
     body: Buffer;
     agentId: AgentId;
     mappedModel: string;
+    sourceModel?: string | null;
   }) => Promise<InterceptedRequest>;
   recordRequestComplete?: (
     intercepted: InterceptedRequest,