Migrate jsonschema v5→v6, eliminate fixSchemaBytes() workaround

[Copilot]

jsonschema/v5 with Draft 7 doesn't support negative-lookahead regexes (Go's regexp package limitation), requiring a ~200-line fixSchemaBytes() runtime patcher. jsonschema/v6 with Draft 2020-12 uses dlclark/regexp2 for ECMAScript regex, eliminating the need entirely.

Schema (mcp-gateway-config.schema.json)

• Bumped $schema to Draft 2020-12
• Replaced negative-lookahead patterns with not: {enum: [...]} equivalents directly in the schema
• Added previously-injected fields (registry, guard-policies, headers) directly — they were only in the schema at runtime via fixSchemaBytes()

validation_schema.go

• Deleted fixSchemaBytes() and newDraft7Compiler()
• Renamed fetchAndFixSchema → fetchSchema (returns raw bytes, no transformation)
• Updated compiler setup: jsonschema.UnmarshalJSON + compiler.AddResource(url, any) (v6 takes a parsed doc, not io.Reader)
• Replaced strings.Contains(ve.Message, ...) fallbacks in formatErrorContext with a type-switch on ve.ErrorKind:

// Before (v5, fragile string matching):
if strings.Contains(msg, "additionalProperties") { ... }

// After (v6, type-stable dispatch):
switch ve.ErrorKind.(type) {
case *kind.AdditionalProperties, *kind.AdditionalItems:
    addFromKeyword("additionalProperties")
case *kind.Required, *kind.DependentRequired:
    addFromKeyword("required")
// ...
}

• InstanceLocation is now []string (joined with / for display); ErrorKind.LocalizedString(schemaErrPrinter) replaces ve.Message

validation.go

• Import changed to jsonschema/v6; AddResource updated to use bytes.NewReader + jsonschema.UnmarshalJSON

Tests

• Deleted fetch_and_fix_schema_test.go (~500 lines testing the removed function)
• Removed 3 fetch tests that validated fixSchemaBytes transformations
• Updated validation_schema_error_format_test.go: ValidationError construction now uses ErrorKind: &kind.X{} and InstanceLocation: []string{...}; TestKeywordFromLocation removed (helper deleted)
• Updated validate_server_against_schema_test.go: compileSchemaForTest uses v6 UnmarshalJSON + AddResource