Emit experimental diagnostics in C# clients - #11685

## Motivation

`@experimental` currently records only that a TypeSpec declaration is
experimental. That is enough for emitters that need a boolean lifecycle
flag, but it does not describe how an emitter should identify the
experiment or whether the generated implementation relies on other
experimental features.

A generated API can be a distinct public experiment while being composed
from several lower-level experiments. For example, an operation tracked
as experiment `C` may internally use experimental features `A` and `B`.
Consumers should see `C`, while an emitter may need `A` and `B` to
generate warning suppressions around implementation code.

## Usage

```typespec
@experimental(#{
  emitterScope: "@typespec/http-client-csharp",
  diagnosticId: "C",
  dependsOn: #["A", "B"]
})
op bar(): void;
```

### Scenarios

- **Direct experiment:** specify only `diagnosticId` when an emitter
needs a stable identifier for the experimental API.
- **Composed experiment:** use `dependsOn` when the generated
implementation consumes other independently experimental features.
- **Emitter-specific diagnostics:** use `emitterScope` when identifiers
are meaningful only to a particular emitter or target language.
- **Lifecycle tooling:** emitters and tooling can query the complete
lifecycle details without parsing decorator syntax.
- **Explicit graduation:** dependencies becoming generally available
does not silently graduate the public API. The declaration remains
experimental until its decorator is explicitly removed.

## API changes

- Add optional `diagnosticId` and `dependsOn` fields to
`FeatureLifecycleOptions`.
- Add `getFeatureLifecycleDetails`, returning:

```ts
{
  stage: "Experimental";
  diagnosticId?: string;
  dependsOn: readonly string[];
}
```

- Preserve the existing `getFeatureLifecycle` API and its
`"Experimental" | undefined` behavior for compatibility.
- Apply existing emitter-scope filtering to all lifecycle details.

## Emitter behavior

The metadata is descriptive rather than prescriptive. Each emitter
decides how to represent it. The C# companion implementation in microsoft#11685
maps `diagnosticId` to `ExperimentalAttribute` and `dependsOn` to scoped
warning suppressions.

## Related work

- C# emitter implementation: microsoft#11685
- Design issue: microsoft#11690

## Validation

- `@typespec/http-client` build and full test suite
- Affected-package lint and formatting checks

Copilot-Session: f0e7f0ac-c4b6-47e3-a4fc-430ff8f883c6