(aws-bedrock-alpha): add BedrockFoundationModel entries for Claude Opus 5 and Fable 5, FoundationModelIdentifier for Mythos 5

### Describe the feature

The Claude 5 family on Amazon Bedrock now includes [Claude Opus 5](https://docs.aws.amazon.com/bedrock/latest/userguide/model-card-anthropic-claude-opus-5.html), [Claude Fable 5](https://docs.aws.amazon.com/bedrock/latest/userguide/model-card-anthropic-claude-fable-5.html), and [Claude Mythos 5](https://docs.aws.amazon.com/bedrock/latest/userguide/model-card-anthropic-claude-mythos-5.html), alongside the already-covered Claude Sonnet 5 (#38271, #38556). None of the three newer models have predefined constants in `FoundationModelIdentifier` (`aws-cdk-lib/aws-bedrock`) or `BedrockFoundationModel` (`@aws-cdk/aws-bedrock-alpha`).

Verified from the model cards:

| Model | Model ID | Bedrock Agents | Cross-region profiles |
|---|---|---|---|
| Claude Opus 5 | `anthropic.claude-opus-5` | Yes | us/eu/au geo + global |
| Claude Fable 5 | `anthropic.claude-fable-5` | Yes | us geo + global |
| Claude Mythos 5 | `anthropic.claude-mythos-5` | No | None (bedrock-mantle only, single region, Preview) |

### Use Case

Referencing the Claude 5 family through the L2 Bedrock constructs the same way as other predefined Anthropic models, instead of hand-constructing model definitions.

### Proposed Solution

- Add `ANTHROPIC_CLAUDE_OPUS_5`, `ANTHROPIC_CLAUDE_FABLE_5`, and `ANTHROPIC_CLAUDE_MYTHOS_5` to `FoundationModelIdentifier` in `aws-cdk-lib/aws-bedrock`, following #38272.
- Add `ANTHROPIC_CLAUDE_OPUS_5` and `ANTHROPIC_CLAUDE_FABLE_5` to `BedrockFoundationModel` in the alpha module with `supportsAgents: true, supportsCrossRegion: true`, following #38038 and #38557.
- Deliberately exclude Mythos 5 from the alpha module: per its model card it supports neither Bedrock Agents nor Invoke/Converse (bedrock-mantle Messages API only, Preview, single region), so a `BedrockFoundationModel` constant would not be usable with the constructs the class serves. The plain identifier in `aws-cdk-lib` still gives users a canonical reference.

Fable 5's docstring should note that using the model requires opting in to the `provider_data_share` data retention mode.

### Other Information

Follow-up to #38556 / #38557, prompted by review feedback on #38557.

### Acknowledgements

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

### CDK version used

2.263.0

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

macOS (Darwin 25.2.0)

Start with FoundationModelIdentifier in aws-cdk-lib/aws-bedrock and BedrockFoundationModel in @aws-cdk/aws-bedrock-alpha, then compare the existing Claude Sonnet 5 entries and the linked model cards. Add Opus 5 and Fable 5 to both appropriate APIs, add Mythos 5 only to FoundationModelIdentifier, and document Fable 5's provider_data_share requirement. Done means the constants expose the listed model IDs and capabilities match the issue.

Written by the indexing model from the issue text.