fix(ros): gate unusable local templates before the first API call

[ruanzhengxin-zhuxing]

Problem

ROS template tools reported an unusable local template as an invalid body_file — a parameter the caller never sent. Callers passing template_url therefore could not tell what to fix from the first failure, and retried the same template input across ros_validate_template, aliyun_api ValidateTemplate, and ros_preview_template before resorting to bash to inspect the file.

Two defects produced that message:

• No pre-call readability gate. Permission checks only authorized the template path, so a missing or non-regular template still returned allow and only failed later, while the wire layer materialized the request body.
• A swallowed error type. _read_body_file converts read failures into ApiContractError("invalid_body_file"), but the local-template branches caught only (OSError, UnicodeError). The error escaped to the generic handler and lost both its template-source context and its structured report.

Observed before the fix — the most common failure mode returned the least diagnostic information:

Local template state	Permission stage	Final error	Structured report
missing	allow	body_file is invalid	none
path is a directory	allow	body_file is invalid	none
not UTF-8	allow	ROS1202 report	ros_validation

Change

Classify local templates (missing / not-regular / unreadable / too-large) before credentials, endpoints, and any request, and report every failure as a ROS1202 diagnostic carrying metadata.ros_validation plus a specific corrective action. The size ceiling is derived from the wire-layer limit so the gate and the transport cannot disagree.

Applied consistently to the runtime, legacy, and ros_stack read paths so the dedicated ROS tools and ros_deploy all fail the same way.

Genuine body_file inputs keep reporting invalid_body_file, and the success path is unchanged.

Verification

• make test: 13282 passed, 4 skipped. Two failures are pre-existing and unrelated — verified failing identically on the untouched base commit d470b40 (a urllib error-class difference and an mtime-granularity ordering assumption).
• make lint (ruff + ty): clean.
• make translate run for the messages domain; the 7 new user-facing strings are translated across zh es fr de ja pt.
• The 6 new regression tests were confirmed to fail when the fix is reverted, so they are not vacuous. They assert the failure names the template source and never body_file, and that no request is assembled (request_builder.inputs == []).