Scope Gateway Target names to their parent Gateway

[aidandaly24]

Description

Gateway Target names should be unique within their parent Gateway, matching the
AgentCore service contract, rather than unique across an entire CLI project.

Today the CLI effectively requires project-global Target names because the L3
construct emits Target outputs using only the Target name:

GatewayTarget<TargetName>Id

The old CLI's CloudFormation output parser also expects that format. As a result,
two Gateways cannot both have a naturally named Target such as search, even
though the service permits it.

Change the L3 output contract to include the parent Gateway:

<GatewayName>Target<TargetName>Id

The CLI should enforce uniqueness only inside each Gateway. During the
transition, the old CLI output parser should accept both the existing
Target-only format and the new Gateway-qualified format so existing deployed
stacks continue to work.

This was intentionally deferred from #2028 to keep that PR
compatible with the currently published L3 output contract.

Acceptance Criteria

• Two different Gateways in one project can each contain a Target with the same name.
• Duplicate Target names within one Gateway are still rejected.
• L3 Target output logical IDs and export names include both Gateway and Target names.
• The old CLI CloudFormation output parser accepts both legacy GatewayTarget<TargetName>Id and new <GatewayName>Target<TargetName>Id outputs.
• Output parsing associates each Target ID with the correct parent Gateway when names are duplicated.
• Existing projects and stacks using legacy output names continue to deploy and update.
• CLI schema, command, L3 synthesis, output-parser, and deployment-state tests cover cross-Gateway duplicate names.
• The project-based refactor removes its temporary project-global Target-name validation after the compatible L3 version is published.

Additional Context

This requires coordinated changes in:

• aws/agentcore-l3-cdk-constructs for Gateway-qualified outputs;
• the old CLI output parser and deployment-state mapping;
• the project-based refactor's Target uniqueness validation.