[API Proposal]: Add ReasoningEffort.Max

### Background and motivation

We maintain a .NET agent harness [dotcraft](https://github.com/DotHarness/dotcraft) that targets multiple providers through one `IChatClient`
pipeline, with reasoning effort as one provider-neutral user setting. Every level up to
`xhigh` rides on `ChatOptions.Reasoning`. `max` cannot, so supporting it means carrying
that one value outside `ChatOptions` and re-shaping the request per provider.

`ReasoningEffort` tops out at `ExtraHigh` → `"xhigh"` (#7319), while both providers with
first-party `IChatClient` adapters now expose a tier above it:

- **OpenAI .NET** — [#1289](https://github.com/openai/openai-dotnet/pull/1289) added
`ExtraHigh` (`"xhigh"`) and `Max` (`"max"`) to `ResponseReasoningEffortLevel`. Merged
2026-08-21, not in a stable release yet (latest 2.13.0).
- **Anthropic .NET 12.42.0** — `Anthropic.Models.Messages.Effort` and its `Beta`
counterpart already expose `Xhigh` and `Max`.

Two providers, same token — not the single-provider case #7318 declined for `Minimal`. And
`ReasoningOptions` is `sealed` with only `Effort` and `Output`, so there is no neutral way
to express it today.

### API Proposal

```csharp
namespace Microsoft.Extensions.AI;

public enum ReasoningEffort
{
None,
Low,
Medium,
High,
ExtraHigh,
Max,
}
```

With the matching arm next to each existing `ExtraHigh` case:

```csharp
// OpenAIResponsesChatClient / OpenAIChatClient
ReasoningEffort.Max => new ResponseReasoningEffortLevel("max"),
ReasoningEffort.Max => new ChatReasoningEffortLevel("max"),
```

String constructor initially, as `"xhigh"` does today; switchable to the generated `.Max`
property once a release containing #1289 is picked up.

### API Usage

```csharp
ChatResponse response = await chatClient.GetResponseAsync("...", new ChatOptions
{
Reasoning = new ReasoningOptions { Effort = ReasoningEffort.Max },
});
```

### Alternative Designs

**`RawRepresentationFactory`.** Works, but requires the caller to know which provider backs
the `IChatClient` and to build the provider-specific request — for a value that is not
provider-specific. It makes the top tier the only effort level that cannot travel through
`ChatOptions`, leaving two parallel paths for one user-facing setting.

**Remap `ExtraHigh` to `"max"`.** Silent behavior and cost change, and `xhigh` becomes
unreachable.

### Risks

_No response_

Start with the ReasoningEffort and ReasoningOptions definitions, then trace the existing ExtraHigh cases in OpenAIResponsesChatClient and OpenAIChatClient. Review how the ChatOptions pipeline carries effort to each provider. Done means Max can be set through ChatOptions and is emitted as "max" by both adapters without changing ExtraHigh behavior.

Written by the indexing model from the issue text.