Enable maximum-strict TypeScript across our packages

tsconfig.base.json adds noUncheckedIndexedAccess,
exactOptionalPropertyTypes, noImplicitOverride,
noFallthroughCasesInSwitch, noUnusedLocals, and noUnusedParameters on
top of strict. Vendored packages opt out of the new flags locally
(their tsconfigs are ours to regenerate; their source is not), keeping
upstream-sync friendliness.

Our code fixed accordingly: index accesses acknowledge undefined
(assembler flush cursors, lastTurnNumber); optional properties are
omitted instead of set-to-undefined (GenerateResult.usage,
ToolDefinition.strict, GenerateOptions.system/tools, error payloads
via an errorData helper); Session.onAppend is explicitly
`(…) => void | undefined`; tests and examples updated for unused
parameters and indexed access.

packages/llm/src/assembler.ts

@@ -123,7 +123,8 @@ export class BlockAssembler {
   flushReady(): ContentBlock[] {
     const ready: ContentBlock[] = []
     while (this.flushed < this.order.length) {
-      const partial = this.partials.get(this.order[this.flushed])!
+      const index = this.order[this.flushed]!
+      const partial = this.partials.get(index)!
       if (!partial.block) break
       ready.push(partial.block)
       this.flushed += 1
@@ -140,7 +141,7 @@ export class BlockAssembler {
   flushRemaining(): ContentBlock[] {
     const remaining: ContentBlock[] = []
     while (this.flushed < this.order.length) {
-      const index = this.order[this.flushed]
+      const index = this.order[this.flushed]!
       remaining.push(this.assemble(this.partials.get(index)!, index))
       this.flushed += 1
     }
@@ -162,6 +163,10 @@ export class BlockAssembler {
 
   /** The assembled non-streaming result. */
   result(): GenerateResult {
-    return { message: this.message(), usage: this._usage, finish: this.finish }
+    return {
+      message: this.message(),
+      ...this._usage !== undefined ? { usage: this._usage } : {},
+      finish: this.finish,
+    }
   }
 }