feat(gateway): surface streaming / streamable-HTTP transport configuration

### Description

AgentCore Gateway supports MCP streamable-HTTP transport on the data plane, but neither the CLI nor `AWS::BedrockAgentCore::Gateway` (CFN) surface any streaming configuration. The CFN [`MCPGatewayConfiguration`](https://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/aws-properties-bedrockagentcore-gateway-mcpgatewayconfiguration.md) exposes only `Instructions`, `SearchType`, and `SupportedVersions` — there is no `Transport`, `StreamableHttp`, or analog property.

Customers building MCP clients want to (a) confirm streaming is enabled, (b) configure transport-level knobs (e.g. heartbeat / keep-alive intervals if any), and (c) see whether streaming is supported on a given gateway via `agentcore status`.

### Acceptance Criteria

- [ ] Confirm with the AgentCore service team which streaming-related parameters are exposed on the data plane (control-plane API, not CFN).
- [ ] If knobs exist on the data plane: add a `streaming` block to `AgentCoreGatewaySchema` (e.g. `streaming: { enabled?: boolean; transport?: 'STREAMABLE_HTTP' | 'SSE' }`) and apply via control-plane `UpdateGateway` (or analog) post-CFN deploy in the same imperative-call pattern used by other post-deploy actions.
- [ ] If no knobs are exposed today: add a doc/banner explaining streaming is enabled by default and link to the MCP spec.
- [ ] Surface streaming status in `agentcore status` for gateways.
- [ ] If/when CFN adds streaming properties, deprecate the imperative path in favor of CFN.

### Additional Context

- CFN support: **NO** — `MCPGatewayConfiguration` does not model streaming. ([docs](https://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/aws-properties-bedrockagentcore-gateway-mcpgatewayconfiguration.md))
- Control-plane: streaming behavior is implicit/internal as of 2026-05-21; needs internal confirmation on whether knobs exist.
- This may collapse to a "documentation-only" issue if there is nothing to configure today.
- Pattern to follow: see #542 (resource-based policies — imperative call pattern for things CFN doesn't yet support).

Bestätige zuerst mit dem AgentCore service team, welche Streaming-Parameter die control-plane API bereitstellt, und verwende dabei die Einschränkung von CFN MCPGatewayConfiguration und die MCP specification als Kontext. Untersuche anschließend AgentCoreGatewaySchema, das imperative-call-Muster nach dem Deployment aus #542 und die Ausgabe von agentcore status gateway. Erledigt ist die Aufgabe, wenn entweder die Streaming-Konfiguration und der Status sichtbar gemacht werden oder das standardmäßig unterstützte Verhalten dokumentiert wird, falls keine Steuerungsmöglichkeiten existieren.

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