fix: widen gateway.domain validation to accept RFC-1123 topology hostnames

[Copilot]

In network-isolation topology mode (gh-aw v0.81.2+), MCP_GATEWAY_DOMAIN is set to a container hostname like awmg-mcpg. Although ${MCP_GATEWAY_DOMAIN} is schema-valid, the validator re-runs post-expansion against the same narrow enum, so the resolved literal awmg-mcpg is rejected and the gateway aborts on startup.

Changes

• validation_schema.go — Add domainHostnamePattern (^[a-z0-9]([a-z0-9-]*[a-z0-9])?$) and accept RFC-1123 single-label hostnames in validateStringPatterns alongside the existing localhost/host.docker.internal/${VAR} forms. Updated error message accordingly.

• schema/mcp-gateway-config.schema.json — Switch domain.oneOf → anyOf (required because localhost matches both the enum branch and the new hostname pattern) and add a third branch for RFC-1123 single-label hostnames. Updated field description.

• validation_schema_test.go / validation_string_patterns_test.go — Add regression cases for awmg-mcpg at both the JSON-schema validation layer and the Go validateStringPatterns layer; update the existing error-message assertion.

After this fix, the following config passes validation where it previously aborted:

{
  "gateway": {
    "port": 8080,
    "domain": "awmg-mcpg",
    "agentId": "${MCP_GATEWAY_AGENT_ID}"
  }
}

[Copilot]

Pull request overview

This PR fixes gateway startup failures in network-isolation topology mode by widening gateway.domain validation to allow RFC-1123 single-label container hostnames (e.g., awmg-mcpg) after ${MCP_GATEWAY_DOMAIN} expansion.

Changes:

• Extend Go-side validateStringPatterns domain validation to accept RFC-1123 single-label hostnames via a new domainHostnamePattern.
• Update the embedded JSON schema to validate gateway.domain using anyOf and a new RFC-1123 single-label hostname pattern branch.
• Add regression tests covering awmg-mcpg at both the schema-validation and Go pattern-validation layers, and update the expected error-message substring.

Show a summary per file

File	Description
internal/config/validation_string_patterns_test.go	Adds comprehensive pattern-validation test cases accepting topology-style single-label hostnames.
internal/config/validation_schema.go	Introduces domainHostnamePattern and expands gateway.domain validation/error messaging to allow RFC-1123 single-label hostnames.
internal/config/validation_schema_test.go	Adds schema + Go-layer regression tests for domain: "awmg-mcpg" and updates the error message assertion.
internal/config/schema/mcp-gateway-config.schema.json	Switches domain from oneOf to anyOf, adds RFC-1123 single-label hostname pattern branch, and updates the field description.

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• Files reviewed: 4/4 changed files
• Comments generated: 0