feat(gateway): session management knobs (timeout, max sessions)

[aidandaly24]

Description

AgentCore Gateway has session semantics on the data plane (each MCP client establishes a session), but AWS::BedrockAgentCore::Gateway does not expose any SessionTimeout, MaxSessions, or SessionConfiguration property. The full Gateway property list is AuthorizerConfiguration, AuthorizerType, Description, ExceptionLevel, InterceptorConfigurations, KmsKeyArn, Name, PolicyEngineConfiguration, ProtocolConfiguration, ProtocolType, RoleArn, Tags — nothing session-related (CFN docs).

The CLI also has no session knobs on AgentCoreGatewaySchema (src/schema/schemas/mcp.ts). Customers in long-running multi-tool sessions cannot set idle timeouts or max session counts.

Acceptance Criteria

• Confirm with the AgentCore service team which session knobs (if any) are configurable today via the control-plane CreateGateway / UpdateGateway APIs.
• If knobs exist: extend AgentCoreGatewaySchema with a sessionConfiguration block (sessionTimeoutSeconds?: number, maxConcurrentSessions?: number, etc.) and apply them via imperative calls during deploy (same pattern used for things CFN doesn't yet support).
• Add --session-timeout-seconds and --max-sessions flags (or the actual subset of knobs supported).
• Surface session config in agentcore status and in the TUI wizard.
• When CFN catches up, deprecate the imperative path in favor of CFN.

Additional Context

• CFN support: NO — Gateway has no session-related properties as of 2026-05-21.
• Control-plane: needs service-team confirmation on which session knobs are exposed; may currently be backend-only/non-configurable.
• This may collapse to "no-op until service exposes knobs" — but the schema slot should still be reserved so we don't break compatibility later.