Introduce `model_capabilities.mantle` flag that installs a provider-specific
override in fast-agent's `ModelDatabase._PROVIDER_MODEL_OVERRIDES` to strip
features the AWS Bedrock Mantle endpoint rejects (beta headers, extended
thinking, task budgets, web tools, prompt caching).
Without this override, fast-agent sends default beta headers and `thinking`
parameters for modern Claude models that Mantle rejects with a misleading
404 "model does not exist" error.
