[MCP]: Let a named screenshot be saved in the output directory

Follow-up to #42487.

### What I would like

I want to give a screenshot a name and still have it saved in the output directory.

Today those two things are exclusive. If I leave `filename` out, the file goes to the output directory but gets a timestamp name. If I give it a name, it goes to the working directory instead. There is no way to have both.

A setting in the Playwright MCP config that makes a named file go under `outputDir` would be enough. Off by default, so nothing changes for anyone who does not turn it on.

```json
{
"outputDir": ".playwright-mcp",
"resolveFilenamesInOutputDir": true
}
```

(The key name is a placeholder.)

### Why

I use the MCP from a coding agent. The working directory is the repository I am editing. The agent names its screenshots, so every screenshot lands at the root of the repository as an untracked file, and I have to clean up after it.

I have two workarounds today. I tell the agent to write `.playwright-mcp/shot.png`, but it does not follow that on every call. I also have a client-side hook that rewrites the name, but it only works in Claude Code; the same MCP used from another client has the same problem. I would rather the server decide where files go, once, in its own config.

### Version

`@playwright/mcp` 0.0.80

Start at the Playwright MCP screenshot entry point and trace how filename and outputDir are currently resolved. Exercise the behavior described in the issue with @playwright/mcp 0.0.80, then add coverage for the opt-in configuration. Done means named screenshots remain in the working directory by default and are placed under outputDir when the setting is enabled.

Written by the indexing model from the issue text.