feat(gateway-target): explicit --listing-mode flag for dynamic tool listing

[aidandaly24]

Description

Dynamic tool listing on a Gateway Target is partially exposed in CFN today via AWS::BedrockAgentCore::GatewayTarget.TargetConfiguration.Mcp.McpServer.ListingMode — allowed values DEFAULT | DYNAMIC. With DYNAMIC, the gateway calls tools/list on the upstream MCP server at request time instead of caching a static list. Source: McpServerTargetConfiguration CFN docs.

The CLI's schema marks toolDefinitions as optional for mcpServer targets (src/schema/schemas/mcp.ts:346–347) — this implicitly maps to DYNAMIC listing, but:

1. There is no explicit --listing-mode flag and no schema field in AgentCoreGatewayTargetSchema that maps to ListingMode.
2. The L3 construct does not pass ListingMode to CfnGatewayTarget; it relies entirely on the presence/absence of toolDefinitions.
3. There is no CFN support for Lambda-backed dynamic listing today — McpLambdaTargetConfiguration only takes LambdaArn + ToolSchema (InlinePayload or S3 — both static). This is a service-side gap.

Acceptance Criteria

• Add listingMode?: 'DEFAULT' | 'DYNAMIC' to the mcpServer discriminator in AgentCoreGatewayTargetSchema.
• Add --listing-mode flag to agentcore add gateway-target --type mcpServer.
• Surface listing-mode toggle in the TUI for MCP server targets.
• L3 construct passes the field through to CfnGatewayTarget.TargetConfiguration.Mcp.McpServer.ListingMode.
• Document that Lambda dynamic listing is NOT supported in CFN (and therefore the CLI) today; track service-team request for that capability.
• Snapshot tests updated.

Additional Context

• CFN support: PARTIAL — McpServer.ListingMode only; Lambda dynamic listing is not modeled.
• L3 gap: yes — @aws/agentcore-cdk Gateway target construct needs to pass ListingMode explicitly.
• Lambda dynamic listing requires a service-side change (no ListingMode on McpLambdaTargetConfiguration); track separately with the service team.
• Reference: McpServerTargetConfiguration says "MCP resources for dynamic targets will be dynamically retrieved when listing tools".