[Feature]: Add optional params to tracing.group() for structured metadata

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### 🚀 Feature Request

Add an optional `params?: Record` field to `tracing.group()` so library users can attach small, structured, machine-readable metadata to a logical trace group.

Today `tracing.group()` accepts a name and optional location, while the corresponding `BeforeActionTraceEvent.params` is always recorded as `{}`. The trace event already supports a params map, and Trace Viewer's Call tab already renders action parameters. Passing optional group params through the existing channel and recorder would therefore make structured group annotations visible without requiring a new trace format or custom viewer behavior.

The proposed change is additive:

- add an optional object to the `tracingGroup` protocol command;
- expose it on the public `tracing.group()` options;
- forward it through the dispatcher and recorder into `BeforeActionTraceEvent.params`;
- document the option and add a hermetic trace assertion.

Existing callers and traces remain unchanged, and older viewers can ignore populated params.

### Example

```ts
await context.tracing.start({ screenshots: true, snapshots: true });

using group = await context.tracing.group('checkout decision', {
params: {
actor: 'shopper-42',
step: 7,
decision: 'continue',
confidence: 0.91,
},
});

await page.getByRole('button', { name: 'Continue' }).click();
```

Selecting the group in Trace Viewer would show these values in the existing Parameters section. Callers would remain responsible for keeping metadata small and excluding sensitive values, as they are for trace titles and attachments today.

### Motivation

String titles are useful for people but brittle for tools. Structured group metadata would support generic use cases such as:

- AI-assisted browser orchestration recording a safe high-level decision category and correlation identifiers;
- custom reporters attaching typed metadata to logical steps;
- observability tooling correlating trace groups with external spans or workflow stages;
- test libraries adding domain-neutral annotations without encoding JSON into group titles.

The underlying event and viewer rendering paths already exist, so this provides a small public seam instead of requiring consumers to overload titles or maintain viewer forks.

Start at the public tracing.group() options and the tracingGroup protocol command, then follow the existing dispatcher and recorder path to BeforeActionTraceEvent.params. Update the documented API and add the requested hermetic trace assertion. Done means optional group metadata reaches the event and appears in the existing Trace Viewer Parameters section while callers without params remain unchanged.

Written by the indexing model from the issue text.