Make omitted Gateway protocolType mean unrestricted

[aidandaly24]

Description

Make an omitted Gateway protocolType consistently mean an unrestricted
Gateway across the old CLI, project schema, and L3 construct.

The current project contract has an unintuitive compatibility mapping:

agentcore.json value	Current behavior
omitted	MCP-only
"MCP"	MCP-only
"None"	unrestricted; L3 omits the service field

The AgentCore service itself uses omission for an unrestricted Gateway and
explicit "MCP" for an MCP-only Gateway. The CLI contract should align:

agentcore.json value	Desired behavior
omitted	unrestricted
"MCP"	MCP-only
"None"	accepted as a legacy unrestricted value

The command experience should be:

# Unrestricted; protocolType omitted
agentcore add gateway --name tools

# MCP-only
agentcore add gateway --name tools --protocol-type MCP

This needs a coordinated compatibility migration. Existing schema-v1 projects
with omitted protocolType historically mean MCP, and imported MCP Gateways are
currently persisted with the field omitted. Those projects must not silently
become unrestricted when the L3 default changes.

This was intentionally deferred from #2028 so that PR can
preserve the currently published schema and L3 behavior.

Acceptance Criteria

• Omitting --protocol-type in the old CLI persists no protocolType field and deploys an unrestricted Gateway.
• --protocol-type MCP persists "protocolType": "MCP" and deploys an MCP-only Gateway.
• The old CLI no longer needs customers to pass --protocol-type None; existing "None" project values remain valid.
• Importing an MCP Gateway persists explicit "protocolType": "MCP".
• Existing projects where omission historically meant MCP are migrated or normalized to explicit "MCP" before consuming the new L3 behavior.
• The L3 treats omitted protocolType as unrestricted and explicit "MCP" as MCP-only.
• Semantic-search protocol configuration remains usable on unrestricted Gateways when supported by the service.
• The old CLI, project-based refactor, schema copies, generated CDK, and L3 tests use the same contract.
• Release sequencing publishes the compatible L3 version before either CLI updates its exact L3 pin.
• Regression tests prove existing MCP projects do not silently widen to unrestricted access.

Additional Context

This is a cross-repository compatibility change involving:

• old CLI authoring, import, migration, generated CDK, and dependency pinning;
• project-based refactor command and schema behavior;
• aws/agentcore-l3-cdk-constructs schema and Gateway synthesis.