fix(adapters): hoist path-template validation into shared forward core, run at from_openapi import (JS-02)

Import-time path-template checks now run on both adapters:
- from_jsonschema: validate_path_template moves to forward.rs (same
  checks, same messages, shared implementation)
- from_openapi: build_registration calls it before building each op, so
  a template like /x{open fails import with 'unterminated placeholder'
  instead of a per-call INTERNAL on first invoke
- new test: unterminated_path_template_fails_import_not_first_call

src/adapters/from_jsonschema.rs

@@ -32,7 +32,7 @@ use async_trait::async_trait;
 use reqwest::Method;
 use serde_json::Value;
 
-use super::forward::{forward, forward_stream, HttpServiceConfig};
+use super::forward::{forward, forward_stream, validate_path_template, HttpServiceConfig};
 use crate::client::SharedHttpClient;
 
 /// The HTTP-backed single-endpoint adapter (ADR-066): one caller-built
@@ -139,30 +139,6 @@ fn validate_method(method: &str) -> Result<(), AdapterError> {
     })?;
     Ok(())
 }
-
-fn validate_path_template(path_template: &str) -> Result<(), AdapterError> {
-    let mut rest = path_template;
-    while let Some(start) = rest.find('{') {
-        let Some(end_rel) = rest[start..].find('}') else {
-            return Err(AdapterError::SchemaParse {
-                message: format!("path template `{path_template}` has an unterminated placeholder"),
-            });
-        };
-        if rest[start + 1..start + end_rel].is_empty() {
-            return Err(AdapterError::SchemaParse {
-                message: format!("path template `{path_template}` has an empty placeholder name"),
-            });
-        }
-        rest = &rest[start + end_rel + 1..];
-    }
-    if path_template.contains('}') && !path_template.contains('{') {
-        return Err(AdapterError::SchemaParse {
-            message: format!("path template `{path_template}` has `}}` without a matching `{{`"),
-        });
-    }
-    Ok(())
-}
-
 fn spec_name_references_undeclared(spec: &OperationSpec, path_template: &str) -> bool {
     let properties = spec
         .input_schema