fix(tools): collapse code-mode executor to run_code for model-direct calls

wireSchemas() already advertised only run_code under mode: 'code', but the
executor resolved every call through get(), which returns the full visible
map plus the reserved transport. A model could name a native tool directly
and bypass run_code entirely. Route the execution-path lookups through a
new private resolveExecution() that applies the mode collapse at the
operation boundary: model-direct calls under 'code' may only name run_code
(UNKNOWN_TOOL otherwise), while SDK sub-dispatches (parent token set) keep
every visible tool. get()/schemas() public semantics are unchanged.

The denial happens at createExecution, before the extensible policy
pipeline — pre-execute listeners, approval ask, and guards never observe
a call that is deterministically denied. A collapsed call honors the
pre-dispatch cancellation contract, routes aborted results through the
visible tool's finalizeContent, and captures the finalizer before
argument materialization.

Under code mode, a system-prompt/assemble listener filters out tool:*
guidance sections that told the model to call native tools directly.
The tools:sdk section and SDK types remain so programs can still use
all tools through run_code.

Fixes #1815

packages/core/tools/tests/tools.spec.ts

@@ -1,5 +1,5 @@
 import { describe, expect, expectTypeOf, it } from 'vitest'
-import { Context } from '@deepseek-ai/cordis'
+import { Context } from 'cordis'
 import { createUserMessage, CallId, HarnessError, type ContentBlock  } from '@deepseek-ai/dsh-llm'
 import SystemPrompt from '@deepseek-ai/dsh-system-prompt'
 import type { Agent } from '@deepseek-ai/dsh-agent'
@@ -907,6 +907,25 @@ describe('ToolRegistry', () => {
     })
   })
 
+  it('keeps the normalized content when the final content transform returns undefined', async () => {
+    const ctx = await setup()
+    let finalized = 0
+    ctx.tools.register({
+      ...echoTool,
+      name: 'identity-finalizer',
+      finalizeContent() {
+        finalized += 1
+        return undefined
+      },
+    })
+
+    const result = await ctx.tools.execute({ signal: testToolSignal, callId: CallId('identity-finalizer'), name: 'identity-finalizer', arguments: {} })
+
+    expect(result.isError).toBe(false)
+    expect(result.content).toEqual([{ type: 'text', text: '' }])
+    expect(finalized).toBe(1)
+  })
+
   it('a block decision can ALSO attach additionalContexts', async () => {
     const ctx = await setup()
     ctx.tools.register(echoTool)