plugin-mcp: update tool registration crashes when schema conversion falls back to permissive Zod schema

Describe the Bug

MCP update tools can fail to register with:

TypeError: convertedFields.partial is not a function

Observed log:

[payload-mcp] ❌ Tool: Update pages Failed to register.
Error [APIError]: Error registering tools for collection pages: TypeError: convertedFields.partial is not a function

### Link to the code that reproduces this issue

N/A

### Reproduction Steps

0. Have the MCP plugin installed 
1. Add a the MCP plugin block to the payload.config.ts with at least one collection.

plugins: [
mcpPlugin({
collections: {
pages: { enabled: true },
},
}),
],

2. Invoke the MCP server, MCP Inspector works as does Codex. 
3. See the error in the server console:

[payload-mcp] ❌ Tool: Update pages Failed to register.
Error [APIError]: Error registering tools for collection pages: TypeError: convertedFields.partial is not a function

### Which area(s) are affected?

plugin: mcp

### Environment Info

```text
Binaries:
  Node: 23.2.0
  npm: 10.9.0
  Yarn: 1.22.22
  pnpm: 10.20.0
Relevant Packages:
  payload: 3.88.0
  next: 16.2.6
  @payloadcms/db-postgres: 3.88.0
  @payloadcms/drizzle: 3.88.0
  @payloadcms/graphql: 3.88.0
  @payloadcms/next/utilities: 3.88.0
  @payloadcms/plugin-mcp: 3.88.0
  @payloadcms/plugin-seo: 3.88.0
  @payloadcms/richtext-lexical: 3.88.0
  @payloadcms/translations: 3.88.0
  @payloadcms/ui/shared: 3.88.0
  react: 19.2.8
  react-dom: 19.2.8
Operating System:
  Platform: linux
  Arch: x64
  Version: #1 SMP PREEMPT_DYNAMIC Thu Jun 18 21:54:43 UTC 2026
  Available memory (MB): 31939
  Available CPU cores: 20