chore: update pnpm to v12 (#872)

Updates pnpm from v11 (`11.20.0`) to v12 (`12.0.0-rc.7`, the latest v12
release — v12 currently ships only pre-releases on the `next-12`
dist-tag).

## Changes

- **`package.json`** — bumped `packageManager` to `pnpm@12.0.0-rc.7`;
this pin drives Corepack and `pnpm/action-setup` (which reads it when no
explicit version is set, as in the CodSpeed workflow)
- **`pnpm-lock.yaml`** — regenerated with pnpm 12

## Lockfile format note

pnpm 12 now writes `pnpm-lock.yaml` as a multi-document YAML stream:

```yaml
---
lockfileVersion: '9.0'

importers:

  .:
    configDependencies: {}
    packageManagerDependencies:
      pnpm:
        specifier: 12.0.0-rc.7
        version: 12.0.0-rc.7
...
---
lockfileVersion: '9.0'
# project dependency graph (unchanged structure)
```

The first document is the new "environment lockfile" pinning the package
manager itself (`packageManagerDependencies`); the second holds the
project dependency graph. `lockfileVersion` stays `'9.0'` and dependency
resolution is unchanged. Tooling that parses the lockfile must read all
YAML documents rather than only the first.

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: fengmk2 <156269+fengmk2@users.noreply.github.com>

Author: Copilot

package.json

@@ -112,5 +112,5 @@
   "engines": {
     "node": ">= 22.19.0"
   },
-  "packageManager": "pnpm@11.20.0"
+  "packageManager": "pnpm@12.0.0-rc.7"
 }