(aws-bedrock-alpha): add BedrockFoundationModel for Claude Sonnet 5

### Describe the feature

The `BedrockFoundationModel` class in `@aws-cdk/aws-bedrock-alpha` ([`bedrock/models.ts`](https://github.com/aws/aws-cdk/blob/main/packages/%40aws-cdk/aws-bedrock-alpha/bedrock/models.ts)) has no predefined constant for Anthropic's Claude Sonnet 5 (`anthropic.claude-sonnet-5`), which is generally available on Amazon Bedrock. The newest Anthropic entries in the file are Claude Opus 4.1 (`anthropic.claude-opus-4-1-20250805-v1:0`) and Claude Sonnet 4 (`anthropic.claude-sonnet-4-20250514-v1:0`).

The core `aws-cdk-lib` `FoundationModelIdentifier` class already gained `anthropic.claude-sonnet-5` (#38271), but the alpha module's `BedrockFoundationModel` was not updated alongside it.

### Use Case

Referencing Claude Sonnet 5 through the L2 Bedrock constructs (agents, cross-region inference profiles) the same way as the other predefined Anthropic models, instead of hand-constructing the model definition.

### Proposed Solution

Add a constant to `BedrockFoundationModel`, e.g.:

```ts
public static readonly ANTHROPIC_CLAUDE_SONNET_5 = new BedrockFoundationModel(
'anthropic.claude-sonnet-5',
{ supportsAgents: true, supportsCrossRegion: true, optimizedForAgents: true },
);
```

Current workaround is instantiating `BedrockFoundationModel` directly with the model ID and capability flags, as above.

### Other Information

Related: #38271 (same request for `FoundationModelIdentifier` in `aws-cdk-lib` `aws-bedrock`, now resolved).

### Acknowledgements

- [ ] I may be able to implement this feature request
- [ ] This feature might incur a breaking change

### CDK version used

2.263.0 (`@aws-cdk/aws-bedrock-alpha@2.263.0-alpha.0`)

### Environment details (OS name and version, etc.)

macOS (Darwin 25.6.0)

Start in packages/@aws-cdk/aws-bedrock-alpha/bedrock/models.ts and compare the existing Anthropic model constants with the related FoundationModelIdentifier change in #38271. Done means BedrockFoundationModel exposes Claude Sonnet 5 with model ID anthropic.claude-sonnet-5 and the capability flags described in the issue, so L2 Bedrock constructs can reference it directly.

Written by the indexing model from the issue text.